Fix #444 Separate recording logic from the concept of a talk/presentation

- Add metadata parameter to load_backend function in Multimedia
- Disable Record button and set tool tip when no presentations are available
- Prepare record_name and metadata in RecordingController and add
  extension to file name in Multimedia
- Split RecordingController load_backend function into two functions: one
  accepting a filename, one accepting a presentation
- Change RecordApp to call load_backend_with_presentation
- Change convenience commands to call load_backend functions from
  RecordingController