Event listeners deleted when moving tabs

[matt1003]

It appears that the view validation that was introduced in commits b5ceb21 and 143a6f8 may be a little too aggressive. It turns out that view.window will be set to None while a tab is being shifted from one position to another. This somewhat makes sense, as the view doesn't really belong to any specific window while it is being moved around.

The problem is that _is_view_valid is invoked while the view is being moved; hence it sees view.window == None and assumes the view is invalid (thus removing the ansi event listeners). As a result, views that have been moved will no longer respond to syntax changes via the sublime menu.