[INTERNAL] ColorIDStore now stores string IDs and not JIDs

All existing Color ID sets are already *vanished* due to a package move
of the JID class. To prevent this for the next time the ColorIDSet now
uses strings as ids and no longer JID objects.

Change-Id: I5cd88e6d0adc8f6a8fdb330f95ea9e4699717d4e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1628
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>