Removed OnInitialize handler from ControlBase

This will never serve it purpose of letting callers add the handler
during creation so I removed it.
Removed stray "Children" List from Grid class

When I implemented the 'Children' interface on the controls I removed
the 'Children' objects directly from each control. Apparently I missed
one.
Added the "AuthoredControl" class

This class allows a game to define their own custom control defined in
its own Gaml file. That file can then be used in any other Gaml file to
bring in and use that custom control as many times as needed.