[3/3] Keyboard dbus

kwin!585 (merged)
plasma-workspace!567 (merged)

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

  All layouts names are now stored in layoutsList Property and not in Action applet's menu objects.
  There is no need for KeyboardDaemon::getLayoutLongName() DBus method any more. See corresponding P-W commit.

  Also fix "&" symbol in ToolTip text.

• fix: keyboard layout applet: no backward scrolling
  Add scrolling in opposite direction.

  Along the way, remove OSD as it doesn't belong here and for consistency with legacy applet. Instead, KeyboardInputRedirection::processKey() should be fixed to have common OSD behaviour for Wayland and X11.

• impr: keyboard layout DBus API: rework to index based
  We don't pass ID in layout list via DBus any more as it's just Index now.

• fix: keyboard layout applet: no flag if Display Name is set
  Reworked for new DBus API and event-driven plugin interface to QML.

  Essentially, we now get only one DBus signal with id when the layout changes.
  All the other info applet is capable to lookup locally via it's own data - no need for additionally DBus calls any more.

  This is possible because new API provides exhaustive info about all layouts via updated getLayoutsList() call - now it returns a vector of all layouts with all names for each. This info is stored on QML side in Actions objects - which at the same time serve to fill context menu.

@davidedmundson