NPE measurement tool develop

see https://trac.openmicroscopy.org.uk/ome/ticket/10495

To test:

 * Open an image, Draw few ROIs.
 * Save.
 * Select a roi
 * Go to the intensity view.
 * Click on the intensity values button.
 * Toggle channels on/off
 * Close the viewer
 * Test 2

* Open an image,
* Open the Measurement tool.
* Do not Select a roi
* Go to the intensity view.
* The intensity values button should be greyed out.
* Select a roi
* The intensity values button should not be greyed out.

--rebased-from #2037