Add `Animation::is_finished`

[vrmiguel]

Hey there :wave: Congrats on this project :smiley:

This PR proposes Animation::is_finished and Animation::has_frames_remaining.

Motivation

This allows us to quickly get rid of "one-off" animations, such as an explosion, damage indicator or whatnot

let is_not_finished = |anim| anim.is_finished().not();

animations.retain(is_not_finished);

[17cupsofcoffee]

Sorry for the massive delay in replying to this, I've been a bit burned out on open source for a while! Finally trying to work through the backlog...

This looks like a sensible feature to add, and low maintainence burden, so I'm fine with merging this :+1:

[17cupsofcoffee]

Thank you!