[async-loading] Update asynchronous image loading

[async-loading] Update asynchronous image loading

Provide a main loop-based fallback to the asynchronous loading in
case the GLib threading support hasn't been enabled. This also
allows us to clean up the asynchronous loading machinery and have
it behave consistently across different scenarios.

Emit the ::load-finished even if the asynchronous loading from
disk was not enabled.

Finally, block clutter_texture_set_from_file() until we have an
image width and height, so that querying the texture actor size
after set_from_file() will still yield the correct result even
when asynchronous loading is set.