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/msw/treectrl.cpp

@@ -31,6 +31,7 @@
 #ifndef WX_PRECOMP
     #include "wx/dynarray.h"
     #include "wx/log.h"
+    #include "wx/app.h"
 #endif
 
 #include "wx/msw/private.h"
@@ -44,7 +45,6 @@
 // comctl32.dll versions
 #define wxUSE_COMCTL32_SAFELY 0
 
-#include "wx/app.h"
 #include "wx/imaglist.h"
 #include "wx/settings.h"
 #include "wx/msw/dragimag.h"