Added "custom" tab to media select widget

Added "custom" tab to media select widget

Added "custom" tab to media select widget.  In this tab, the user can
launch the template designer or delete an existing user-defined template.
Added notification callbacks to libglabels db subsystem so that the
media select widget will automatically update when user-defined templates
are added or deleted.  The "edit" button does not do anything yet.

Also did some visual cleanup to the new label dialog.

Also include ../libglabels in SUBDIRS in src/ so that libglabels can
be rebuilt from src/ directory.