After transitioning to FileSystemAccessServer, some of the methods
in `MainWidget` and `ProjectLoader` for opening files directly with
a filename as opposed to with a file descriptor are unused. This
commit removes them.
The fd would get closed when the File went out of scope, so we couldn't
open any file specified by 'pp <path to file>'. We need the fd to be
alive and we solemnly swear to take good care of it and close it
ourselves later.
This new class will open and parse files (either images directly or .pp
project files) and one can get the parsed Image as well as other
information from it.
This patch removes a bunch of 'try_create_from..." methods from Image in
favor of using the ProjectLoader.
The only json_metadata that is available are Guides for now.
