[#7023] Add the ability to load an abstract contact backend in the library to resolve more data, polish code

Fix memory leaks in client
Change a bit dbus protocol: int -> int instead of int -> string -> string -> int
Remove unused and empty yamlengine

[#7021] Refactor KDE client dbus handling, add a missing call in daemon and port the DataEngine to the new API

- update scripts
- complete french translation (100%)
- move gnome po files into one directory
- update po files to current state

It make more sense to keep it as it was. Un the case of a Dataengine, Canonical AppIndicator / KDE Plasmoid systray RFC, the daemon can run event if the client have not been openned or have been closed.

This reverts commit 66a2b8ee.