Coalesce wxEVT_TEXT events in wxGTK wxTextCtrl and wxComboBox

For consistency with the other platforms, coalesce multiple wxEVT_TEXT
events resulting from a single user action into a single one in wxGTK
too. For example, when pressing a key in a control with some text
selected, wxGTK previously generated 2 wxEVT_TEXT events: one
corresponding to the removal of the selection and another one to the
addition of the new text. Now only a single event with the new text is
generated, as in the other ports.

Doing this requires delaying sending wxEVT_TEXT until GTK itself ends
handling the key press, however we delay it as little as possible, so
hopefully this shouldn't have any visible effects at wx API level.

Closes #10050.

src/gtk/textctrl.cpp

@@ -858,7 +858,11 @@ GtkEntry *wxTextCtrl::GetEntry() const
 int wxTextCtrl::GTKIMFilterKeypress(GdkEventKey* event) const
 {
     if (IsSingleLine())
-        return wxTextEntry::GTKIMFilterKeypress(event);
+        return GTKEntryIMFilterKeypress(event);
+
+    // When not calling GTKEntryIMFilterKeypress(), we need to notify the code
+    // in wxTextEntry about the key presses explicitly.
+    GTKEntryOnKeypress(m_text);
 
     int result = false;
 #if GTK_CHECK_VERSION(2, 22, 0)