In qview, the plot tables should no longer have multiple rows with the same X...

In qview, the plot tables should no longer have multiple rows with the same X value. Also, fixed a crash related to closing the plot table windows. Fixes #710. Fixes #818.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5007 41f8697f-d340-4b68-9986-7bafba869bb8