fix lib UI

[p3r7]

lib/ui was broken on latest seamstress due to 2 reasons:

• a typo in UI.Tabs (see https://github.com/ryleelyman/seamstress/commit/9b5bc40206779f4d4b6d87dc12730c5a34cf012a)
• calls to removed functions screen.set / screen.reset

this PR solves #2 rather drastically, by removing the notion of window a UI element can draw itself to. the other approach would have been to replace screen.set & screen.reset w/ calls to _seamstress.screen_set.

that's specifically where i would need some guidance. i interpreted "discouraged" as deprecated in your comment:

breaking change: screen.set is removed; drawing to the params window in your scripts is discouraged.

[robbielyman]

Hmmm. I guess the only thing that gives me pause is that it would actually be very nice to use the UI library in the params menu. what about this: keep window out of the doc comments, but do keep it as a function param and just use _seamstress.screen_set?

[p3r7]

alright, i'll switch to the _seamstress.screen_set implem.

[p3r7]

done

[robbielyman]

thanks for revising! and for catching that changes were necessary!