Allow to add RESET values from existing keys

[KoBeWi]

This PR adds a way to quickly create RESET values from existing animation keys. It's useful when importing old animations or when you just want to update the RESET value to match another animation.

If multiple keys are selected, only the first selected key in a track will get a RESET value:

Closes #55634

[akien-mga]

Thanks!

[securas]

Nice! Thanks!

[akien-mga]

@KoBeWi I did a backport for 3.x in my local branch, and it seems to work, but it raises an error when adding a reset track this way:

 scene/resources/animation.cpp:881 - Index p_track = 0 is out of bounds (tracks.size() = 0).

This is the commit in my fork (didn't push to upstream 3.x yet): akien-mga@10d200c

I didn't check if it's reproducible in master.

[KoBeWi]

I don't know how I missed this error, I'll make a hotfix >_>

[akien-mga]

Cherry-picked for 3.5. (Together with #55671.)