Changed set separator to <b>&cup;</b>.

It looks awesome and it only appears between two sets or more.
Simplified tagged document filter bypass.

The inital approach was not bad but this looks much simpler and more intuitive.
Implemented remove-button from #keep-selection.

It probably needs more tests but it seems to do what it's supposed to.
Everything is explained in the comments abundantly and the script itself
doesn't look too bad. I've seen worse.