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

- Add filename parameter to load_backend function in RecordingController
- 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
- Change convenience commands to call load_backend from RecordingController