Fix creating disabled windows in wxGTK too

Don't forbid calling Enable() before creating the window and just do
nothing in this case and do disable the window when it's actually
created if it's supposed to be disabled.

Note that this doesn't work for classes overriding Enable() directly,
such as wxButton or wxCheckBox, currently.

src/gtk/window.cpp

@@ -2913,6 +2913,11 @@ void wxWindowGTK::PostCreation()
 
     SetLayoutDirection(wxLayout_Default);
 
+    // if the window had been disabled before being created, it should be
+    // created in the initially disabled state
+    if ( !m_isEnabled )
+        DoEnable(false);
+
     // unless the window was created initially hidden (i.e. Hide() had been
     // called before Create()), we should show it at GTK+ level as well
     if (m_isShown)
@@ -4196,7 +4201,12 @@ bool wxWindowGTK::IsShown() const
 
 void wxWindowGTK::DoEnable( bool enable )
 {
-    wxCHECK_RET( (m_widget != NULL), wxT("invalid window") );
+    if ( !m_widget )
+    {
+        // The window can be disabled before being created, so just don't do
+        // anything in this case and, in particular, don't assert.
+        return;
+    }
 
     gtk_widget_set_sensitive( m_widget, enable );
     if (m_wxwindow && (m_wxwindow != m_widget))