QgsTask subclasses can now return a result (success or fail)
directly from QgsTask::run. If they do so then there's no
need for them to manually call completed() or stopped()
to report their completion.
Alternatively, tasks can also return the ResultPending value
to indicate that the task is still operating and will
manually report its completion by calling completed() or
stopped(). This may be useful for tasks which rely on external
events for completion, eg downloading a file. In this case
Qt slots could be created which are connected to the download
completion or termination and which call completed() or
stopped() to indicate the task has finished operations.
Now all evaluate/prepare/etc methods must be called using QgsExpressionContexts
Also remove most remaining traces of special variables. This brings some
user facing changes, such that existing expressions may need to be
updated if they used these old special variables (eg $scale,
$feature). These changes are noted in doc/qgis3_user_changes.dox
so that we can include them in the release notes.
* pyqtwrappers update (add QtNetwork, QtXml, QtSql, QtTest, uic)
* 2to3 updates
* move QPyNullVariant/NULL to PyQt.QtCore
* add global unicode/basestring/long for Python3
* expand QtGui, QtCore module and star exports
* Qscintilla2
* replace Set import with set builtin
