Should Tween._done be a list?

[tianzhuqiao]

Thanks for the great project, @shakiba. Currently Tween._done supports one function. Should we make it a list, such that the following application will also work?
img.tween(100).alpha(0).remove().done(fn)

Otherwise, it will fail to remove since

1. remove() set _done to this.remove(),
2. but done(fn) will overwrite the _done. to fn.

[shakiba]

Hmm, yes that doesn't look good. I'm not sure if we should queue or replace multiple "done", but remove and done should be separated. Would you be able to make a PR to fix it?

[tianzhuqiao]

Yeah, separate remove() and hide() from done() is a good idea.