[FIX] SharedProjectDecorator did not properly update labels

The updating was not working correctly for full shared projects
as we do not know the resources of a full shared projects.

Just simplified the logic so a full refresh is made when projects
are added or the session stopped.

Change-Id: Ib44f7279cbe7fac65fcd23e35801acd095bc8567
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1120
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>