AssetManager: separate addAsset

Separate addAsset from updateTask. This allows to extend AssetManager and adding assets without using a task (e.g. manually loaded assets to display a loading screen). It could even have a public version, but that requires more checks (like #load does).

On a side note: I also have a modified version of AssetManager to allow (but not force) specifying different loaders based on filetype. Are you interested in such a change?