More tags box fixes (follow-up to f932f312ebbc)

Use the Notifier for all tags box updates to ensure that it always updates.

Also fix the tag count and some other things.

Post-tab focus still isn't 100% correct in all situations, but it's real close.
Fix glitch entering tag with whitespace

If an existing colored tag was added to an item with whitespace around it, the
tag didn't immediately show up with a color.
Disable key navigation on 1-6 keys if any tag colors are assigned

Previously any unused numbers were still available for key navigation, but I
think it's too easy hit the wrong key that way, and getting moved to another
part of the items list is annoying.
Fixes incorrect post-edit scrolling

Fixes #268, broken in 2cd45be19162d

Post-edit scrolling can probably be smarter, but this fixes the brokenness.
Merge branch '3.0' into 4.0