[Spread] remove unused includes

- also sort includes
- also additions to precompiled header

src/Mod/Spreadsheet/App/SheetObserver.cpp

@@ -21,9 +21,11 @@
  ***************************************************************************/
 
 #include "PreCompiled.h"
+
 #include "SheetObserver.h"
 #include "PropertySheet.h"
 
+
 using namespace Spreadsheet;
 using namespace App;