... specifically from KisPart.h, which itself is included from a lot of
source files directly or indirectly. This change drastically reduce
rebuild times after modifying KisMainWindow.h.

This includes introducing the methods
KisPart::currentMainwindowAsQWidget() and
KisViewManager::mainWindowAsQWidget(). These allow getting the main
window for use as a parent of dialogs without needing to include
KisMainWindow.h.