MainWindow: set minimum size to 640x400

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Make HUD the center widget in Flight view. Add menu item for map widget.
hud is a tool in VIEW_FLIGHT
MainWindow: handle maptool widget destroy event

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>