Filtering by tags should be toggle-able

[ian-r-rose]

1. Select a tag.
2. Press "Filter". The cells without that tag are removed.
3. Press "Filter" again. Nothing happens.
4. Unselect the tag. Nothing happens.

I found this behavior confusing. I think it makes more sense to have filter be a toggle-able item. I'm not sure how to describe the current behavior.

[kenjioman]

Related to this -- if all tags are removed while a tag-based filter is in place, some kind of a dialog should be available to clear the filter. Currently, toggling the tags button just show there are no tags available, with none of the document structure visible.

Steps to reproduce:

1. Add a tag to a cell for a tag-free notebook (no other tags in any of the cells for the notebook)
2. Toggle filtering on that tag (so, only that cell will show up in the TOC)
3. Remove the tag on the cell

Now, we don't see anything in the TOC, and there is no indication that tag-based filtering is taking place. I didn't try testing this to see if closing/ re-opening the notebook would clear this, but in any case, there would ideally be an indicator to notify the unsuspecting user that tag-based filtering is in place, but there are currently no tags, so users can be given the option to clear the active filter.