[Gui] So headers: remove unused includes

- also sort out some headers to be used in precompiled headers
- also move a boost header to precompiled headers

src/Gui/Selection.cpp

@@ -24,11 +24,10 @@
 #include "PreCompiled.h"
 
 #ifndef _PreComp_
+# include <boost/algorithm/string/predicate.hpp>
 # include <QApplication>
 #endif
 
-#include <boost/algorithm/string/predicate.hpp>
-
 #include <App/Application.h>
 #include <App/Document.h>
 #include <App/DocumentObject.h>