Load characters (and any other Prefab) from Tiled

Game Designers can now specify what objects are loaded in a map directly from Tiled.

They do this by adding Properties to objects:

`0-prefab` - The name of the Prefab (e.g. Brute)
`0-prefab path` - The path to the prefab from */Resources* with a trailing slash (i.e. prefabs/enemies)

Note: Prefabs have to be located under the /Resources folder in order for the Tiled importer to find them (due to a limitation in Unity - FindResources is the API call). We should eventually migrate over all assets into Resources, methinks.