Improve MainWindow state save/restore

Rather than tracking the visibility manually, we now make sure that the window state is saved
before the window is hidden, i.e. not in aboutToQuit() which is already too late for that.
Instead of calling QApplication::quit(), we use our new slot MainWin::quit() which saves the window
layout first.

For KDEified Quassel, we now use KMainWindow's autosave feature rather than using QMainWindow::restoreState().
This might fix some issues with multiple desktops etc, though I haven't tested that. In any case it's cleaner
as it saves additional properties (e.g. toolbar state).