LP#1832897: add Angular widget to for selecting multiple linked rows

LP#1832897: add Angular widget to for selecting multiple linked rows

This component provides a widget to allow the user to select
multiple linked rows. In particularly, it is meant to handle
IDL fields whose underlying database columns are intarrays that
refer to records in another IDL class.

The widget's user interface consists of an eg-combobox for selecting
new values to add to the list and a list of the existing values.

The component has the following attributes:

- idlClass: IDL class of the records being linked to
- linkedLibraryLabel: if supplied, specifies that the display
  label in the comboox should include the library shortname as
  found in the specified field.
- startValue: init value to display

This component emits onChange events.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>