Channel truncated develop

Same as gh-1961 now on develop

To test:
 * open an image in Full Viewer
 * open the "Display Rendering Controls" (opens the right-hand pane in the Full Viewer)
 * still in the Full Viewer, go to the right-hand pane you just opened, General Tab
 * find the channel names, edit one and put in for example "very long name centre very long name" press "Save"
 * Make sure the name is updated on channel buttons and the name on the General Tab is updated
 * Also try to enter  a very long name.  Limited to 256 characters now

--rebased-from #1961