When using multiple monitors, the resolution can change in OSX,
during plug-in/off off the display(s). To avoid window becoming
non-usable, there is EnsureVisible, that is called in several
places.
However, if you minimize VeraCrypt on the bigger screen, and
restore it in the main screen of a MacBook, after unplugging, the
window will become unusable (unless you know that clicking 'About'
calls EnsureVisible :-)).
Call EnsureVisible OnActivate so MainFrame is always functional
across screens, even when minimized.
