Presets: Support discarding old textures

[snailspeed3]

When we apply a preset--especially, via a bulk action--the prior (pre-application) textures (and animations) remain in the scene tree. This is clutter and a filesize problem (for textures)--and probably a crash issue for animations.

We should add a checkbox/setting to prune previously-referenced (and now dead) resources as a result of the "apply preset" action. Some care should be taken, since multiple materials can reference the same texture.