[Feature request] Highlight amount of data in cache

[0n-s]

The mpv cache is automatically used for what it automatically detects as "slow media" (see mpv(1) on --cache).

mpv's OSC highlights how much data is remaining in this cache like this (the white line & the top right):

Celluloid lacks this useful feature.

I imagine that if this feature request is accepted, it will be somewhat intertwined with the in-progress UI redesign, so as an aside I will also note that the mpv OSC progress bar is of a much nicer size than Celluloid's & fits much better with this guideline from GNOME's HIG: "Click targets should be large enough to be comfortably used with different pointing devices and physical abilities."