MainWindow: store the window maximized state

We add the "maximized" entry for the settings group "MainWindow",
and store it on close, so that the window state is the same on
the next run. But then also, we only store the window size and resize
to that size if not maximized. This attempts to preserve a "restored"
window size.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>