More responsive menus

[po5]

I've been thinking about our conversations in #13 and the need for a secondary script to do non-blocking processing.
In some ways, that's correct. But I just had an idea that doesn't involve extra mpv instances or extra script files, while still allowing for real parallel operations! The concept works in the limited tests I've done so far, I need to update memo to work with it.
The vanilla UI will of course benefit the most, but it will also be great for responsiveness because it will allow cancelling uosc searches when the query changes.

If the memo experiment works I'll definitely implement it in other scripts that do lengthy blocking operations, or need very fast feedback (thumbfast?).