Implement Activities Runner as DBus Runner
This MR implements the activities runner using the DBus API. The runner allows one to view and switch to other activities.
Because the data is already available in the kactivitymanagerd process it makes very little sense to have a KRunner plugin which fetches this data and produces the results when we can provide this functionality in the actual running process.
See https://phabricator.kde.org/T12031 for more discussion about the topic in general.
PS: The .desktop file was copied from the old runner and the xml+dbus utils file are just boilerplate.