New content creation options and features.

[Menotdan]

This adds several new features to the content creation workflow.

1. Creating particle systems from particle emitters. This simply creates a new particle system asset with a track containing the particle emitter asset.
2. Creating animation graphs from skinned models. This simply sets the base model of the animation graph to the skinned model asset the graph was created from. This feature is currently a bit hacky because visject, it opens an animation graph editor window to set the base model and then save that, this avoids reusing code but the behavior may seem weird to users, maybe leave the window open after the operation.
3. Picking the root asset type of a prefab:
   
4. Picking the root control type (or canvas) for a widget:
   
   

This also adds showing prefabs which have a UICanvas or UIControl as the root as "Widget" instead of "Prefab", to match how the UI Editor works. Currently this is cached because its potentially slow as I load the prefab into memory and create a default instance of it, but that could break when people change whether or not a prefab is a "widget" or not, since it won't update.

This PR is based off of #1132.