qutebrowser: Prevent proliferation of sessions

[ayekat]

Right now I'm looking at 49 qutebrowser sessions, of which I maybe use 3 or 4 regularly, and probably 10 in total. The rest are either obsolete, or just due to typos.

Maybe the acts of creating and restoring sessions should be split; in particular, restoring a non-existing session should not be possible. This would prevent typos (which probably account for about 30% of all the sessions).

Also (or alternatively) having an elegant way to list and delete existing sessions would be nice, too.