Forward key down/up events from wxCompositeWindow children too

There doesn't seem to be any reason to only forward CHAR events, but not
KEY_{DOWN,UP} ones, so do the same thing for the latter ones too.

This allows to get rid of GetMainWindow() call in wxDataViewCtrl unit
tests, as wxEVT_KEY_DOWN are now correctly received in the control
itself and not just its main window.

tests/controls/dataviewctrltest.cpp

@@ -423,7 +423,7 @@ TEST_CASE_METHOD(SingleSelectDataViewCtrlTestCase,
     if ( !EnableUITests() )
         return;
 
-    EventCounter keyEvents(m_dvc->GetMainWindow(), wxEVT_KEY_DOWN);
+    EventCounter keyEvents(m_dvc, wxEVT_KEY_DOWN);
 
     m_dvc->SetFocus();