Call EnsureVisible() for OSX in OnActivate (#1167)

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.

src/Main/Forms/MainFrame.cpp

@@ -737,6 +737,7 @@ namespace VeraCrypt
 #ifdef TC_MACOSX
 		if (event.GetActive() && Gui->IsInBackgroundMode())
 			Gui->SetBackgroundMode (false);
+		EnsureVisible();
 #endif
 		event.Skip();
 	}