Streamline URI schemes
Introduce nautilus-scheme.h, a simple header that contains all schemes known to nautilus.
Then replaces the URI related functions in eel-vfs-extensions.h and nautilus-file-utilities.h as discussed in #3032 (closed).
There are currently many commits that should be squashed IMO, I kept them separate in case it makes reviewing easier.
I also believe the check uri scheme via GFile commits should be squashed, probably into general: check URI schemes via GFile.
This fixes #3032 (closed).