markers: Add beat detection support
Depends on !402 (merged) getting merged first.
This MR implements the main goal of my GSoC project - beat detection.
Uses 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.