Add extension point for the 'Included In' drop-down panel

The new extension point allows plugins to add additional rows in the
'Included In' drop-down panel on the change screen.

E.g. if there is a fixed set of production servers a plugin can show
on which servers this change was deployed. To provide this information
the plugin could e.g. ask each production server which tag it runs and
then include the name of this server if the change is included in this
tag. Being able to immediately see on which production servers a
change is running is very useful for users to know if a bug-fix has
reached a certain server or which servers are effected by a bad
change.

Change-Id: I93ba27fe84e6830f485b5117afe9c951e8dc885a
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
8 files changed