Fix wxGetKeyState() on non-X11 wxGTK backends (e.g., Wayland)

wxGetKeyState() does not currently work on non-X11 GTK backends, and in some
cases it has been reported to crash.  It seems that the most likely use case
for wxGetKeyState() is to query the modifier keys, so on non-X11 backends, use
GTK+ calls to retrieve the modifier key state.

Non-modifier keys are not currently implemented, update the documentation to
mention this.

Closes https://github.com/wxWidgets/wxWidgets/pull/320

interface/wx/utils.h

@@ -372,6 +372,9 @@ wxString wxGetDisplayName();
     Even though there are virtual key codes defined for mouse buttons, they
     cannot be used with this function currently.
 
+    In wxGTK, this function can be only used with modifier keys (@c WXK_ALT, @c
+    WXK_CONTROL and @c WXK_SHIFT) when not using X11 backend currently.
+
     @header{wx/utils.h}
 */
 bool wxGetKeyState(wxKeyCode key);