Library support for per-activity pinned tasks

- The URL in the configuration file now contains also the list
  of activities it should be shown in
- The configuration format is backwards compatible
- Added API to differentiate between the shown launchers (for
  the current activity), and all configured launchers
- Supports reordering of the launchers

Testing: It needs the ivan/per-activity-launchers branch of
plasma-workspace because of the API changes