Clear the inspector tab.

To test this PR
* Open an image 
* Open the Measurement tool
* Draw few rectangle on the same plane
* Save
* Select the inspector tab
* Modify color for example and hit Save
* Check that the inspector tab is cleared up
* Select another ROI, hit save again. The inspector tab should be cleared up

See http://trac.openmicroscopy.org/ome/ticket/12393