liblightdm-gobject: use libxkbcommon, libxkbregistry instead of libxklavier

[jpeisach]

libxklavier is "considered deprecated". This pulls keymap information from the registry, and then changes to the current layout are done via setxkbmap.

TODO

• Recommit the PR with a shorter name
• Add the "since" tags or whatever for the new "variant" layout prop
• Figure out how to clean up the.. abundance.. of variants that did not show up by default for active keyboard layouts
• Formatting
• Update CI
• Clean up as much as possible

LLM Disclosure

Use of search result responses to help debug and find information. Most notably helped me realize I needed the x11_xkb bridge extension thing.

Closes #415