Don't re-register an already registered action in QgsShortcutsManager

Avoids incorrect warnings about duplicate shortcuts on startup.

What's happening here is:
- on QGIS startup, plugins are loaded, adding their actions to
the interface via iface.registerMainWindowAction()
- after ALL plugins and qgis native menus and actions are created,
the shortcut manager registers ALL children from the main window.
This includes the actions and widgets created by plugins, which
have already been registered to the manager.
- There's no way to avoid this duplicate registration - we could
move the child shortcut registration to occur before plugin
initialization, but it's actually nice to have this "catch-all"
occur after plugins are loaded (so that plugins which don't
correctly register actions still have them included in the shortcut
manager). Similarly, plugins MUST use the registerMainWindowAction
call instead of just relying on the Qt QAction.setShortcut method
because otherwise the shortcuts manager is unaware of actions
created after QGIS load - e.g. enabling a plugin after startup.

So we avoid this by just refusing to re-register a shortcut
that the manager is already aware of... no more startup warnings!

src/gui/qgsshortcutsmanager.cpp

@@ -77,6 +77,9 @@ void QgsShortcutsManager::registerAllChildShortcuts( QObject *object, bool recur
 
 bool QgsShortcutsManager::registerAction( QAction *action, const QString &defaultSequence )
 {
+  if ( mActions.contains( action ) )
+    return false; // already registered
+
 #ifdef QGISDEBUG
   // if using a debug build, warn on duplicate actions
   if ( actionByName( action->text() ) || shortcutByName( action->text() ) )

tests/src/python/test_qgsshortcutsmanager.py

@@ -90,6 +90,11 @@ class TestQgsShortcutsManager(unittest.TestCase):
         action2 = QAction('action2', None)
         action2.setShortcut('y')
         self.assertTrue(s.registerAction(action2, 'B'))
+        self.assertCountEqual(s.listActions(), [action1, action2])
+
+        # try re-registering an existing action - should fail, but leave action registered
+        self.assertFalse(s.registerAction(action2, 'B'))
+        self.assertCountEqual(s.listActions(), [action1, action2])
 
         # actions should have been set to default sequences
         self.assertEqual(action1.shortcut().toString(), 'A')