Add editing for camera focus (basic)

[leveleditor]

In some levels the camera starts out focused on a specific tile, building, or unit.

(Note: this is closely related to indicators, however this issue is only for a basic one-time focus at level startup. Advanced focus like lock/follow would require indicator editing (future todo)).

This is going to require giving each unit/building a unique ID for targeting purposes (a string ID/name would work: only letters/numbers/underscores, no spaces, no duplicates).

Tiles are already unique and can be targeted by their x/y position.