When showing a file dialog, Qt can choose to use the "system" file
dialog, which will make QgsExternalResourceWidget to loose the focus.
This patch blocks all the events that are sent to
QgsExternalResourceWidget while a dialog is shown, this way it will keep
the focus until the dialog is closed.
Sponsored by the QGIS project (qgis.org)
Fixes: #26948
This should apply to both PROJ < 6 and >=6 builds.
Fixes the following error reported by Valgrind
```
==5848== Invalid read of size 4
==5848== at 0xE5E4195: internal_pj_ctx_get_errno (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0xE5E16AE: internal_pj_free (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0xE5F4E78: internal_proj_destroy (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F961A7: QgsProjUtils::ProjPJDeleter::operator()(PJconsts*) (qgsprojutils.cpp:76)
==5848== by 0x7D2D44C: std::unique_ptr<PJconsts, QgsProjUtils::ProjPJDeleter>::~unique_ptr() (unique_ptr.h:239)
==5848== by 0x7D2C9BD: QgsCoordinateReferenceSystemPrivate::~QgsCoordinateReferenceSystemPrivate() (qgscoordinatereferencesystem_p.h:94)
==5848== by 0x7D2D6FA: QExplicitlySharedDataPointer<QgsCoordinateReferenceSystemPrivate>::~QExplicitlySharedDataPointer() (qshareddata.h:165)
==5848== by 0x7CDFA77: QgsCoordinateReferenceSystem::~QgsCoordinateReferenceSystem() (qgscoordinatereferencesystem.cpp:233)
==5848== by 0x55CC0AF: QgsEllipsoidUtils::EllipsoidParameters::~EllipsoidParameters() (qgsellipsoidutils.h:39)
==5848== by 0x7D8C40B: QHashNode<QString, QgsEllipsoidUtils::EllipsoidParameters>::~QHashNode() (qhash.h:149)
==5848== by 0x7D8C43F: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::deleteNode2(QHashData::Node*) (qhash.h:536)
==5848== by 0x9BE3B78: QHashData::free_helper(void (*)(QHashData::Node*)) (in /opt/qt59/lib/libQt5Core.so.5.9.1)
==5848== by 0x7D8C466: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::freeData(QHashData*) (qhash.h:576)
==5848== by 0x7D8CCED: QHash<QString, QgsEllipsoidUtils::EllipsoidParameters>::~QHash() (qhash.h:254)
==5848== by 0xA60E369: __cxa_finalize (cxa_finalize.c:56)
==5848== by 0x77BE742: ??? (in /home/even/qgis/QGIS/build/output/lib/libqgis_core.so.3.9.0)
==5848== by 0x4010DF6: _dl_fini (dl-fini.c:235)
==5848== by 0xA60DFF7: __run_exit_handlers (exit.c:82)
==5848== by 0xA60E044: exit (exit.c:104)
==5848== by 0xA5F4836: (below main) (libc-start.c:325)
==5848== Address 0x45db80e0 is 0 bytes inside a block of size 136 free'd
==5848== at 0x4C2F440: operator delete(void*) (vg_replace_malloc.c:586)
==5848== by 0xE5F5A3A: internal_proj_context_destroy (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F9617A: QgsProjContext::~QgsProjContext() (qgsprojutils.cpp:48)
==5848== by 0xA60E5FE: __call_tls_dtors (cxa_thread_atexit_impl.c:155)
==5848== by 0xA60DF26: __run_exit_handlers (exit.c:40)
==5848== by 0xA60E044: exit (exit.c:104)
==5848== by 0xA5F4836: (below main) (libc-start.c:325)
==5848== Block was alloc'd at
==5848== at 0x4C2E709: operator new(unsigned long, std::nothrow_t const&) (vg_replace_malloc.c:387)
==5848== by 0xE5E445E: internal_pj_ctx_alloc (in /home/even/proj/install-proj-master/lib/libproj.so.15.2.0)
==5848== by 0x7F96151: QgsProjContext::QgsProjContext() (qgsprojutils.cpp:39)
==5848== by 0x7F970A9: __tls_init (qgsprojutils.cpp:31)
==5848== by 0x7F970F3: TLS wrapper function for QgsProjContext::sProjContext (in /home/even/qgis/QGIS/build/output/lib/libqgis_core.so.3.9.0)
==5848== by 0x7F96186: QgsProjContext::get() (qgsprojutils.cpp:57)
==5848== by 0x7D86B13: QgsEllipsoidUtils::definitions() (qgsellipsoidutils.cpp:351)
==5848== by 0x7D863FE: QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}::operator()() const (qgsellipsoidutils.cpp:173)
==5848== by 0x7D87EE1: void std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (functional:1531)
==5848== by 0x7D87BC5: std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()>::operator()() (functional:1520)
==5848== by 0x7D879A2: void std::__once_call_impl<std::_Bind_simple<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1} ()> >() (mutex:706)
==5848== by 0x10A6AA98: __pthread_once_slow (pthread_once.c:116)
==5848== by 0x7D863CE: __gthread_once(int*, void (*)()) (gthr-default.h:699)
==5848== by 0x7D8787F: void std::call_once<QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}>(std::once_flag&, QgsEllipsoidUtils::ellipsoidParameters(QString const&)::{lambda()#1}&&) (mutex:738)
==5848== by 0x7D86476: QgsEllipsoidUtils::ellipsoidParameters(QString const&) (qgsellipsoidutils.cpp:174)
==5848== by 0x7E8D572: QgsMapSettings::setEllipsoid(QString const&) (qgsmapsettings.cpp:322)
==5848== by 0x6A4E5F1: QgsMapCanvas::QgsMapCanvas(QWidget*) (qgsmapcanvas.cpp:143)
==5848== by 0x5110841: QgisApp::QgisApp(QSplashScreen*, bool, bool, QString const&, QString const&, QWidget*, QFlags<Qt::WindowType>) (qgisapp.cpp:803)
==5848== by 0x417DB9: main (main.cpp:1339)
==5848==
```
