feat: runtime configurable keybindings

[mmstick]

Work in progress. Adds key bindings to cosmic-comp-config to enable configuring it at runtime in the settings application.

[Drakulix]

Looks good on first glance, however I was hoping we could eventually move this out of cosmic-comp, see pop-os/xdg-desktop-portal-cosmic#4 (comment) .

Essentially given global-shortcuts will need to be managed somehow (including permissions, remember configuration, etc) by our portal, I would prefer another process to be responsible for this and cosmic-comp just forwarding registered events. (And also not call things like amixer, but let the audio applet register it's key-bind or cosmic-workspaces/applibrary/etc.)

But I suppose we can move that code later.

However if the only thing the config still contains in the end is data_control_enabled we could drop the old config. (Though I don't want to make that variable hot-reloadable and easily user-writable as that is a security hole. So we probably would want to make that an environment variable.)