* Moved entirely into MainWindow.
* Moved duplicated code into separate methods.
* Resize the main window on larger screens by default, as we can make
use of the extra space.
* Use BWindow::MoveOnScreen() instead of make_sure_frame_is_on_screen()
as the former has more info. And is even smarter now as it can
optionally resize windows to fit on screen.
* Center window on screen by default (ie. when there are no settings).
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