[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/SoFCSelectionContext.cpp

@@ -19,13 +19,18 @@
  *   Suite 330, Boston, MA  02111-1307, USA                                 *
  *                                                                          *
  ****************************************************************************/
+
 #include "PreCompiled.h"
-#include <boost/algorithm/string/predicate.hpp>
-#include <Inventor/misc/SoState.h>
-#include <Inventor/elements/SoGLCacheContextElement.h>
-#include <Inventor/elements/SoCacheElement.h>
+
+#ifndef _PreComp_
+# include <boost/algorithm/string/predicate.hpp>
+# include <Inventor/elements/SoCacheElement.h>
+# include <Inventor/misc/SoState.h>
+#endif
+
+#include "SoFCSelectionContext.h"
 #include "SoFCUnifiedSelection.h"
-#include "Selection.h"
+
 
 using namespace Gui;