Made Overview window less jumpy with its updates
Newly created gibs now correctly follow hull
Added 'Follow Hull' button to gibs
Changed Restore event to be sent by ShipContainer

...instead of the restored controller. Allows the event to contain
information about the controller's position in the ship's hierarchy,
thus allowing to restore the object to its old index.

Since OverviewWindow needs to listen for Restore event to update its
contents, it had to be changed to a persistent singleton, so that it can
receive events even when the window is not opened (widgets are
instantiated only when the user opens the window)
Fixed offset undo when modified by shift-dragging