Add IM and full wxEVT_CHAR support to wxTextCtrl and wxComboBox in wxGTK.

Generate wxEVT_CHAR events for non-ASCII characters entered in these controls
by intercepting their insert-text signal.

Also try to use GtkEntry/GtkTextView internal IM objects but unsuccessfully so
far.

Closes #3158.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73695 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/textctrl.h

@@ -166,6 +166,10 @@ protected:
 
     virtual void DoSetValue(const wxString &value, int flags = 0);
 
+    // Override this to use either GtkEntry or GtkTextView IME depending on the
+    // kind of control we are.
+    virtual int GTKIMFilterKeypress(GdkEventKey* event) const;
+
     virtual wxPoint DoPositionToCoords(long pos) const;
 
     // wrappers hiding the differences between functions doing the same thing