[2/3] Keyboard dbus

kwin!585 (merged)
plasma-desktop!293 (merged)

• 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.

• fix: keyboard layout applet: wrong scroll direction

  Make mouse wheel layouts scrolling direction in a more natural way - corresponding to layouts order in context menu

• impr: keyboard layout DBus API: rework to index based

  Accomodate API changes on server sides.
  ID is not marshalling over DBus any more as it's just Index now.

• fix: keyboard layout applet: no flag if Display Name is set

  Rework DBus API - getLayoutsList() now provides full details about the layouts.
  Remove Q_PROPERTY's as they are not make much sense here.
  Port to a fully event-driven interface towards QML.

@davidedmundson