Support querying `systemd-localed` for keyboard layout

[phkaeser]

As observed in #495 , not all distros have a /etc/default/keyboard configuration file to read.

Accoding to Gemini lore, the recommended modern way of retrieving the system's global keyboard configuration is from systemd-localed, through the D-Bus interface.

busctl get-property org.freedesktop.locale1 /org/freedesktop/locale1 org.freedesktop.locale1 X11Layout

And retrieve X11Layout as well as the other parameters:

• X11Model
• X11Layout
• X11Variant
• X11Options

Request: Add D-Bus client for querying these values.

[megatux]

Is the dependency on Systemd based distros something to document on build/requirements?

[phkaeser]

The current implementation does not require a Systemd based distro. I am currently thinking that the mentioned D-Bus client would be an optional build dependency -- there is always the option of configuring the keyboard settings in the config file.

(I am wondering if there is a common approach to define the keyboard layout other than Systemd and other than /etc/default/keyboard, though...)