Added Sequences and Spawners to Tween class

[eligt]

This allows creating very complicated tweening branching trees in a
readable way without having to manage delays manually.

Sample (functional) GDScript code looks like this:
https://github.com/eligt/godot/wiki/Tweening-sequences-and-spawners-sample-GDScript-code

Creates this resulting animation:
https://youtu.be/AVkn0ri0oHw

[akien-mga]

Moving to 3.2 milestone as we're about to enter the beta stage and release freeze for Godot 3.1. Only bug fixes against issues of the 3.1 milestone (or enhancements to core features of the 3.1 roadmap) will be considered until 3.1-stable is released.

Note: If it's merged after 3.1-stable is released, this change might be considered for cherry-picking into a later 3.1.x release.

[eligt]

I've noticed this is marked for the 3.2 milestone and I know that 3.2 is to be released relatively soon so I've updated the branch, merging it with master. I've also done a fair bit of testing (using the Tween Demo project) and everything seems to be working as expected.

[realkotob]

If this is still considered for 3.2 I can run some tests to see if it still runs fine on the master branch.

The changes seem pretty isolated so they shouldn't really have any weird side effects.

[lentsius-bark]

Guess this won't make it to 3.2, hope we get a tween upgrade soon. I liked this code as the samply gdscript looks very readable.

[MarcusElg]

@eligt There are a few conflicts rn. Mind fixing them?

[aaronfranke]

@eligt Is this still desired? If so, it needs to be rebased on the latest master branch.

If not, abandoned pull requests will be closed in the future as announced here.

[aaronfranke]

This PR has not received any new commits for almost a year and is abandoned, closing.