add animation

[sindu12jun]

Hi, I have made animation a default fault option config

[ErickLuis00]

Containers without items do not work the animation and break the shadow

Solution:

Before
if (!mover) { return; }

After
if (!mover) { mover = dropTarget }

Thanks for making the animation, it helped a lot in my project.

[sindu12jun]

Containers without items do not work the animation and break the shadow

Solution:

Before
if (!mover) { return; }

After
if (!mover) { mover = dropTarget }

Thanks for making the animation, it helped a lot in my project.

OK thanks a lot, I have updated my PR