gscan: Added stop and release menu items

[dvalters]

Closes #2227

Fixed bug where menu icons didn't display properly.

[hjoliver]

To close #2227 we need to be able to hold or stop suites en-masse, by right-clicking on the group name. If I right-click on a group on this branch I get:

Traceback (most recent call last):
  File "/home/vagrant/cylc/lib/cylc/gui/gscan.py", line 260, in _on_button_press_event
    self.HOST_COLUMN, self.OWNER_COLUMN, self.SUITE_COLUMN)
TypeError: iter must be a GtkTreeIter

Also, although it could be another PR, indicate held suites somehow in gscan (this may not be obvious from the task state summary, until all active tasks have finished).

[dvalters]

Last commit to this PR should now allow stopping, holding, and releasing suites en-masse when right-clicking the group name. (It doesn't state held suites in gscan though.)

[hjoliver]

Thanks @dvalters, very nice (I haven't reviewed the code yet, but it certainly works well).