file: refactor icon loading

Separate things out in different functions, and avoid exposing emblems
as public API.
file: remove unused icon flag

This is not needed anymore.
file: apply emblems to all gicons

Including the default icons.
file: move icon thumbnailing code to a separate function

This will help to finally clean up nautilus_file_get_icon()
file: remove unnecessary ref/unref
file: move some code around

We'll need this in the next commit.
file: apply emblems to thumbnails too
file: consolidate some code