`Timeline` as a `Component`

[nixon-voxell]

Currently, the way we reference the animation playback time is through the Timeline struct, which derives from Resource. This means that in a single Bevy ECS world, there can only be one Timeline.

This limitation is a little hard to work with when animators wants to create multiple segmentations of animations that can be played with different criteria (e.g. speed, starting time, play direction, etc)

With the introduction of Timeline as a Component, animators will be able to create multiple Timelines in a single Bevy ECS world, therefore, removing all of the limitations mentioned above!