Unify custom/transient object creation and move to catalog system

[brickbots]

With the addition of manually entered RA/DEC objects, we now have two different ways of adding arbitrary, transient objects

• pos_server.py - The LX200 server uses the goto command to add new objects to the catalog system.
• ui/radec_entry.py - Manual entry of RA/DEC

It seems like all of this should be integrated into the catalog system somehow. Perhaps by adding a nice way to create new objects in the catalog class and starting each session with a blank 'custom' or 'user' catalog which then receives both the push and entered objects.

This could also be the place where we try to match the RA/DEC against our DB to see if we have an existing object to re-use.

[mrosseel]

Agreed, this will be in the extra catalog feature (wip)