Keep track of Cleared watches and prevent them from being added back to the watch tree on subsequent FSI submissions

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The Clear Watches and Clear All functions are currently one-shot actions: 
previously cleared watches are added back to FsEye on subsequent FSI 
submissions. We should be able to track previously cleared (or individually 
removed) watches and prevent them from being added back in.

Note that it may be useful to add a complementary feature that "unclears" 
previously cleared / removed watches.

Also note that resetting the FSI session acts as a "hard" clear, since FsEye 
can only maintains watches for the current FSI session.

Original issue reported on code.google.com by [email protected] on 21 Jul 2013 at 10:53