GIF/Video Scrubbing

[ShadowJonathan]

Tie-in with #31, and after seeing #171;

I'd like to be able to have swipe-scrubbing with GIFs, like how Apollo had it, because it was literally the only app or software that even offered it, and its very important in the era of sharing GIFs and such online.

[ShadowJonathan]

We might be able to do this once we have a handle on getting a GIF to scrub to a particular frame, then we can tie-in a swiping gesture to essentially scrub backwards and forwards across GIFs real quick.

[EricBAndrews]

I'm restricting the scope of this issue to focus on the backend infrastructure needed to make this work across different animated image types:

• Create unified scrubbing API through MediaControlState
• Handle scrubbing in each media type
• Link up to simple frontend control

More sophisticated scrubbing UI is tracked in #1834