Don't assert if Ctrl+Tab is pressed inside a wxGTK window

Ignore the WinChange flag in DoNavigateIn() to fix an assertion that happened
when pressing Ctrl+Tab with the generic wxListCtrl having focus.

This seems to be consistent with pressing Ctrl+Tab elsewhere, e.g. when
cycling between buttons in a dialog.

(this is a backport of f3cd79984f2936a4148964af8c23cd42aab6a871 from master)

src/gtk/window.cpp

@@ -3712,14 +3712,6 @@ void wxWindowGTK::DoMoveInTabOrder(wxWindow *win, WindowOrder move)
 }
 
 bool wxWindowGTK::DoNavigateIn(int flags)
-{
-    if ( flags & wxNavigationKeyEvent::WinChange )
-    {
-        wxFAIL_MSG( wxT("not implemented") );
-
-        return false;
-    }
-    else // navigate inside the container
 {
     wxWindow *parent = wxGetTopLevelParent((wxWindow *)this);
     wxCHECK_MSG( parent, false, wxT("every window must have a TLW parent") );
@@ -3733,7 +3725,6 @@ bool wxWindowGTK::DoNavigateIn(int flags)
 
     return rc != 0;
 }
-}
 
 bool wxWindowGTK::GTKWidgetNeedsMnemonic() const
 {