impr: Keyboard Layout plugin: drop excessive DBus API method arguments

Considerations:
* In any point of time, the only layout info client might need is about:
    - a single layout (the current one)
    - list of all layouts configured (e.g to fill context menu entries)
  There is no chance it might need to asqure some random layout info
  which is not current.
* To get current layout Display Name, client shouldn't have to asqure
  the current layout itself first - such dependecy makes code based on
  async DBus API unnecessary complicated.
* Shall the layouts context menu be ever desired, it would be easy to
  implement by adding both Name lists (Short/Long) and Index getters to
  the API.

KWin/P-D parts:
impr: Keyboard Layout DBus API: drop excessive method arguments