Added support for Ctrl-A selection, row deletion with the delete key, and the...

Added support for Ctrl-A selection, row deletion with the delete key, and the context menu only appears when a right-click occurs on a selection.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2935 41f8697f-d340-4b68-9986-7bafba869bb8