markers: Add beat detection support
Depends on !402 (merged) getting merged first.
This MR implements the main goal of my GSoC project - beat detection.
librosa as the beat detection backend, leverages
GstTranscoder to transcode audio into the WAV format that we're sure
librosa will correctly parse.
Adds a basic UI section to
ClipProperties with a "beat intensity" setting (higher intensity -> more beats shown), as well as detect / clear buttons.