kcms/activities: Clean-up and port to an actual QML module

The Activities KCM used to be a QWidgets KCM with some embedded QML
components: the activity list and the activity editor. There were
also some unnecesary settings and many leftovers of unused code.

UI Changes:
- Remove the VirtualDesktop switching setting, which was already
  getting dropped from KActivitiesManagerD.
- Remove the switching Shortcuts settings, as it isn't even complete,
  and can be better accessed from the Shortcuts KCM
- Move the "confirm deletion prompt" to QML side
- Move the "Create New" action to the header
- Move the activity editor into a page of the main KCM.

Code Changes:
- Remove all the KConfigXT settings, since none of those were handled
  in this KCM, but in the recentfiles one.
- Remove the custom private plugin and custom paths to interoperate
  with QML: we can use the KCM goodies for that
- Remove the XML DBus interface. Just use DBus messages
- Port to new kcmutils_add_qml_kcm function
- Use the KCM arguments to edit a specific activity

i18n: Port to standard translation domain kcm_activities

Remove XML DBus interface