Make the tag widget act more sanely when pressing Tab

When dealing with autocompletion, tag usually means "take this, move on".
In the tag widget the tab was added to the tag itself (and then stripped
when the input line was processed). Not exactly useful.

This feels a bit "hackish", but it seems to get the job done.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>