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/cocoa/combobox.mm

@@ -81,6 +81,7 @@
 #ifndef WX_PRECOMP
     #include "wx/window.h"
     #include "wx/log.h"
+    #include "wx/app.h"
 #endif // WX_PRECOMP
 
 #include "wx/cocoa/ObjcPose.h"
@@ -159,7 +160,6 @@ void wxCocoaNSComboBox::DisassociateNSComboBox(WX_NSComboBox cocoaNSComboBox)
 
 @end // implementation wxPoserNSComboBox
 
-#include "wx/app.h"
 #include "wx/combobox.h"
 
 #include "wx/cocoa/autorelease.h"