Support animations in VoxelModel assets

[RiscadoA]

Problem

Currently the VoxelModel asset has no support for animations, although some voxel model formats, like .vox, support it.
It is cumbersome to manually construct animation keyframes from individual models when that information is already present in the file.

Solution

The Voxel Model asset should also store keyframe information, if available. How it would be stored should be discussed further before implementing it.