Hide the drag notifications when the inspector is destroyed via the global event. Fixes issue https://bugs.launchpad.net/juju-gui/+bug/1282125
Merge pull request #137 from hatched/inspector-mask-1282125

Fixes issue https://bugs.launchpad.net/juju-gui/+bug/1282125

__To QA__
Follow the steps in the bug to reproduce issue, if the issue isn't there then it's resolved. 

__Note__
This code will be removed shortly when the inspector mask code gets moved inside the new inspector class.