Audio file info display and "media" predicate

[mnzaki]

This PR adds some UI improvements to audio content blobs, and adds a "media" predicate that searches through all media tags.

Related to #202

Some context:
I am hacking on the audio file handling a bit as I want to use perkeep to play music from my collection on my phone, and just wanted to share this early on. The next thing I wanted to do is have a central player element instead of a per-file one, so that I can navigate while it's playing.

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[mnzaki]

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[MichaHoffmann]

Is this PR still alive? I would also like to store my audio files on perkeep and find the UI elements and tag searchability a little lacking. Its working with raw queries, but a little cumbersome.