Optional xkb config paths should be included

### What Happened?

Configuration files in `/etc/xkb` or `~/.config/xkb` are never read. It looks like XkbLayoutHandler.vala reads only a single `evdev.xml` file (defaulting to the system one in `/usr/share/X11/xkb/rules`).

### Steps to Reproduce

1. Add a new layout or variant in `/etc/xkb` or `~/.config/xkb`. Test it using xkbcli so you know it's valid.
2. Freshly open keyboard settings and try to select the new layout. It won't be there.

### Expected Behavior

Keyboard layouts added to one or both of the two other locations searched by libxkbcommon and related tools should be supported.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_