Use F3 to centre any map object, not just topics

F3 centres and focuses on a selected topic, but does not work for anything else.

It could also be used to centre background objects and groups.

Currently, if a (named) background object is selected in the Elements tab in the Map Index, the object is selected but only gets scrolled into view if no part of it is visible. If any part of it is visible, it is only selected, not scrolled. It would be handy to press F3 to centre the object.