Remove contextual menu when right clicking on non-post rows in the posts view.

The actual cause of the crash is when "Edit Post" is clicked. However,
none of the menu items apply to an empty row, thus don't bother showing
the menu.

No further code added other than making TBPostsViewController a NSMenuDelegate and handling menuNeedsUpdate.

I'm unsure if hiding all the items is the "correct" solution, but it works.