Added KoMainWindow::viewFullscreen(bool) to be able to switch between fullscreen and normal modes.

The method does not more then calling showFullScreen()/showNormal() atm but could be later extended to e.g. hide dockers at fullscreen. IMHO it would make sense to introduce a way to differ between different modes and there view-settings to be able to say that e.g. in fullscreen-mode only docker "xyz" should be displayed while in normal-mode others should be displayed. Anyway, that's todo for later :)

To add it to your ko-app, just put a <Action name="view_fullscreen"/> into your apps *.rc file and in the view do something like

actioncollection->addAction("view_fullscreen", view->shell()->actionCollection()->action("view_fullscreen"));