Updated documentation on qisis Control class. Declared Control pointer as Qt...

Updated documentation on qisis Control class. Declared Control pointer as Qt metatype for use with QVariant. Fixes #3949.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@6750 41f8697f-d340-4b68-9986-7bafba869bb8