Enable arbitrary timelines for GIF and Video assets

[Zulko]

Motivation

Right now users can only tweak the speed at which a Video or GIFs will play. Operations like freezing on one frame, playing backwards, or progressively slowing down, are not possible.

Specs

• Users can check "Use an arbitrary timeline" in the editor (linked to a new usesCustomTimeline in the Asset schema). This disables the "speed factor" and "time crop" elements of the forms, and instead creates a form similar made of two columns Project Time and Asset Time, which enables the users to say "this time in the project corresponds to that time in the asset".
• The gix-renderer is updated so that the frame returned for that asset will take into account the custom timeline when it is defined.