Add delete menu to file explorer

[jeffyoung]

In this PR, we've removed the automatic promoting of file deletions from candidate to pending changes. This should address #241.

Tested scenarios:

1. At command line, delete a versioned file via filesystem, re-create file with same name, shown as EDIT, defaults to included
   • move between excluded and included groups, works? yes
   • run "undo", works? yes
2. At command line, delete a versioned file via filesystem, file shows in excluded changes, works? yes
   • move to included list (tf delete should get run on it), works? yes
   • move to excluded list, behaves as expected, works? yes
   • move back to included list (tf delete should NOT be run on it again), works? yes
3. Use "Delete (TFVC)" menu item, item ends up in included list, works? yes
   • move to excluded list, works? yes
   • move back and forth between the two lists, works? yes
4. Use "Delete (TFVC)" menu item, item ends up in included list, run undo, changes are undone? yes
5. 'tf delete' does not show up on F1, works? yes

[jpricket]

What happens if the deletes are just included? does the checkin fail?

[jeffyoung]

If it's the only file in the Included section, yes. Basically, it's a 'detected change' and the check-in fails with this:

[Checkin] No changes were matched by any arguments.

Presumably, if there are other files in that Included list, those would get checked in. Without this change, if the file is versioned and they delete it via the command prompt or explorer, it would appear in the Included list but not be checked in. With this change, it'll be excluded and when they move it to included, we'll run tf delete on it.

[jpricket]

Can we check ahead of time and just do a normal delete for them? I.e. so the user can ALWAYS call our delete.

[jeffyoung]

We certainly could but then we'd want to also do it for rename. I just followed the rename pattern (if the file isn't in the workspace, we won't run a "tfvc" command on it). I don't have passion either way although we could do this and see if we get any feedback. Thoughts?

[jpricket]

Makes sense. Let's keep it this way and get user feedback.

[jpricket]

Couple comments but looks good!