refactor: keyboard layout applet: return to Properties-based QML API

Return "layout" and "layoutsList" Properties where all layouts names are
now stored.
getLayoutLongName() DBus method is not used any more so it was also
deleted from DBus API in KWin and P-D, see corresponding commits there.

The Properties interface allows QML code be more declarative.
There is a trade-off though as now we have to:
- store all layout names on client side, even if it's not needed for
base QML layout component here
- duplicate layouts in layoutsList Property as they are also stored in
Action objects of the plasmoid in P-D.