[RFC] Add action binder protocol

Hello! since !56 (closed) got closed a month ago, i'd want to try to keep developing this!

while most attention has pivoted to the global shortcut portal, i still think that having a wayland native way to bind actions is important.

it allows for clients to work without needing dbus, which can be very useful for environments that wish to stay simpler, and also for simple compatibility layers such as xdg-desktop-portal-wlr.

i'm completely new on wayland development so i'd like some feedback on this

• Review
• ACKs
• • TBD
• • TBD
• Implementations
• • wlroots: wlroots/wlroots!4386
• • TBD

previous discussing in https://patchwork.freedesktop.org/patch/172113/ and !56 (closed)