feat: expose keyboard layout Short Name via DBus

Due space constraints, Short Name is the only name suitable for keyboard
layout indication on panels and systray. Usually it's just 2 symbols
corresponding to standard ISO country code.

libxkbcommon doesn't have this information, so we have to store it in
compositor for the exposing:
https://github.com/xkbcommon/libxkbcommon/issues/192

It's exposed by getLayoutDisplayName() DBus method now, as it should initially.
For Long Name, getCurrentLayoutLongName() method was added.

Relevant P-W commits:
Keyboard Layout plugin: passthrough Short Name from compositor to QML applet
fix: Keyboard Layout plugin: wrong property for passing Short Name
X11 part, P-D:
feat: expose keyboard layout Long Name via DBus

CCBUG: 390079
FEATURE: