Switch from threading to asyncio

[wmww]

What we were doing before was wildly unsafe, and I think may have been introducing some random failures (especially with the virtual pointer tests I've been working on)

[Saviq]

Something's not right still, the amount of frames captured is too low, at least for the asciinema case and neverputt:

test	frame count
test_active_app[asciinema-confined-shell]	1
test_active_app[mir-kiosk-neverputt-confined-shell]	90
test_compositor_alone[confined-shell]	5
test_inactive_app[qterminal-confined-shell]	5
test_inactive_app[gedit-confined-shell]	27
test_inactive_app[mir-kiosk-kodi-confined-shell]	22

[Saviq]

Something's not right still, the amount of frames captured is too low, at least for the asciinema case and neverputt:

The neverputt case could be a software rendering issue (I get over 500 frames), but asciinema I can reproduce the "1" locally.

[bors[bot]]

Build succeeded:

• ProcessSnaps
• RunTests (self)