Move functionality that's not widget specific out of QtWidgets

The QtWidgets module includes several components that are not widget specific, and that should be moved out into a separate module, into QtGUI, or into QtCore.

Recommendations:

• Run a Qt Creator's clang checks on the code base first to identify (fix in Qt 5)
  • Static invocations: Fix qApp->sendEvent() and similar to QCoreApplication::sendEvent() (reduce dependencies on QApplication)
  • Use member initialization in private classes to reduce constructor #ifdefery
  • Check the autotest of the class and port to Qt 5 signal slot connections and ideally also fix nullptr to get rid of things like (Class *)0.
• On no account introduce workarounds forward-declaring QtWidget classes from QtGui . This creates huge problems for Qt for Python.
• Test source compatibility by compiling Qt Designer

In conjunction with the QAction removal: it might make sense to move QShortcut out (that is, extract a QAbstractShortcut into QtGui) - depending on whether that makes sense for Quick controls (Friedemann, 6/19).