NPE measurement tool

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.