Add support for listening and reconfiguring the keyboard settings from locale1

Weston currently only supports statically configuring the keyboard from weston.ini(5), but nowadays there is a good source for keyboard settings that Weston could listen to: org.freedesktop.locale1.

Please add support for it so that things like login managers and keyboard layout configuration tools that set the keyboard through org.freedesktop.locale1 would be able to configure Weston dynamically.

Here are a couple of example implementations for other compositors:

• KWin: https://invent.kde.org/plasma/kwin/-/merge_requests/2921
• Sway: https://github.com/alebastr/sway-systemd/blob/main/src/locale1-xkb-config