Support D-Bus RPC on Linux

[ananace]

It would be useful if livesplit-core could serve a D-Bus service with methods for accessing the same functionality as the hotkeys or the websocket feature in LiveSplitOne, which would make integrations with macropad software - and the like - that can perform D-Bus calls much more seamless, as they wouldn't have to emulate a keyboard or mess about with hosting websockets in order to perform local low-latency actions.

This is probably slightly related to #619 - since implementing XDG hotkey portal support would require the addition of a D-Bus client as well.