broadcast: allow overlapping inheritance

This allows broadcasts on families to be used with broadcasts on a task.
tests: remove unnecessary line in suite
cylc gui: remove hardcoded colours
cylc gui: remove commented out line
Merge pull request #883 from matthewrmshin/broadcast-inherit

broadcast: allow overlapping inheritance
Merge pull request #886 from benfitzpatrick/cylc.gui-change-colours

cylc gui: remove main window hardcoded colours