In qview, qtie and qmos the stretch tool wasn't handling switching viewports...

In qview, qtie and qmos the stretch tool wasn't handling switching viewports (selecting different cubes) correctly. The min/max stretch values weren't updating, and the advanced stretch changed stretches that it shouldn't have. These problems are now fixed. Fixes #771.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4557 41f8697f-d340-4b68-9986-7bafba869bb8