Utilize extra information from audio file header

### Enhancement Summary
Audio files can include additional non-audio data such as loop points, time signature, tempo, etc. This information can be useful in many places such as AudioFileProcessor and sample tracks.
- [ ] Read useful non-audio data from audio files
- [ ] Utilize the data
- [ ] AudioFileProcessor: loop settings
- [ ] AudioFileProcessor: base note
- [ ] Sample track: tempo and time signature, once we implement time stretching in sample tracks?
- [ ] Display the data?

#### Justification
There has been several suggestions for utilizing this kind of information: #2154, #5583, #5634, #6542.
Libsndfile provides APIs for handling several properties of a audio file, see https://github.com/LMMS/lmms/issues/2154#issuecomment-122207578.

Start by reviewing the existing AudioFileProcessor and sample-track entry points, then inspect the libsndfile APIs linked in the issue for available metadata. Break the work into the listed metadata-reading and usage areas, and confirm which properties are supported before choosing a scope. Done should include a defined subset of metadata, its intended consumers, and tests or visible behavior for that subset.

Written by the indexing model from the issue text.