v0.8.3 — One shelf at a time, and a download you cannot order twice

You could order the same download twice

After a download finished, Check for updates came back to life next to Restart and
install — and pressing it painted over the ready-to-install panel and offered to fetch the
same 55 MB again. Taking that offer threw away the copy already on disk and re-downloaded
the whole thing, as many times as you liked.

Fixed on both sides. The backend now refuses a download while one is already staged, and the
button follows the state instead of being switched back on unconditionally.

The staged panel also gained a Discard download button. Keys has always been able to
throw away a staged update — it is 88 MB sitting beside the application, and declining it
should be possible — but nothing on screen ever offered, so the only ways out were installing
it or restarting.

Instruments and Profiles are a toggle now

The Instruments panel showed both shelves stacked, which made it two scrollers tall and
neither of them a comfortable height. They are also two different questions: what sound do I
want means browsing 287 of them, load the rig I built means picking one of a handful.

They are now a toggle, with Instruments showing by default. The panel is a third shorter,
which also makes it a much better neighbour on Play's first row.