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/x11/palette.cpp

@@ -34,9 +34,16 @@ recommended; only the window manager should do this...  Also, it is
 not the functionality that wxPalette::Create() aims to provide.
  */
 
+// for compilers that support precompilation, includes "wx.h".
+#include "wx/wxprec.h"
+
 #include "wx/palette.h"
+
+#ifndef WX_PRECOMP
+    #include "wx/app.h"
+#endif
+
 #include "wx/window.h"
-#include "wx/app.h"
 #include "wx/utils.h"
 
 #ifdef __VMS__