Add customisable hotkey binds

[anone-zone]

This PR moves hotkey handling out of backends and adds configuration via lua. Adds support for issue #457.

[misyltoad]

Hmmm, this has a decent amount of overlap with fef0bca

I wonder if these two things can be merged. We should also make this configurable from lua.

[anone-zone]

Oh neat, I can have a go at frankensteining something together

[anone-zone]

Alright, I've ported the code to use the gamescope_action_binding protocol, and added configuration via lua.

Couple things I'm still unsure about:

We probably want default keys + commands defined somewhere in cpp-land in case the lua config file doesn't exist.

Starting another thread for hotkeys feels icky, but wlserver has to be running in order to bind actions, and I wasn't sure where else to put it.

[misyltoad]

Can we make this not just execute a command, but just run a lua function that can do that?

[anone-zone]

Added a lua function for running ConCommands.

Btw looks like pipelines are failing due to extra/wlroots getting renamed to wlroots0.18.
Resolved in 5a15c24.