FR: Shield impact animation configurability

[Kestrellius]

There is currently no way to configure basically anything about a shield impact animation except the frames themselves.

Requested options:

• Effect duration, as float. (This is the important one; all others are secondary.)
• Size factor, as float.
• For duration and size, random ranges.
• Random rotation.
• Ability to list multiple image sequences and have one randomly selected every time the animation is played.