Include wx/app.h according to precompiled headers of wx/wx.h (with other minor cleaning).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38945 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/frame.cpp

@@ -19,9 +19,13 @@
 // ----------------------------------------------------------------------------
 
 #include "wx/frame.h"
+
+#ifndef WX_PRECOMP
+    #include "wx/app.h"
+#endif // WX_PRECOMP
+
 #include "wx/dialog.h"
 #include "wx/control.h"
-#include "wx/app.h"
 #include "wx/menu.h"
 #if wxUSE_TOOLBAR
     #include "wx/toolbar.h"