Remove header reliance on QWidget
Although KWindowSystem does link to widgets, this should not provide otherwise 'pure' QtGui consumers with compile errors from including headers. This patch removes QWidget reliance from xcb/netwm, and header reliance from kwindowffects. kwindowsystem.h was already free.
Note there are still forward declarations present (QtGui/qwindowdefs.h provides them in addition to WId in any case), as QWidget* is part of a few of the API functions.