Implement a process-local VFS to enable grouping together arbitrary
URIs passed via program arguments, DnD, or the file open dialog.
This VFS contains FivCollectionFile objects, which act as "simple"
proxies over arbitrary GFiles. Their true URIs may be retrieved
through the "standard::target-uri" attribute, in a similar way to
GVfs's "recent" and "trash" backends.
(The main reason we proxy rather than just hackishly return foreign
GFiles from the VFS is that loading them would switch the current
directory, and break iteration as a result.
We could also keep the collection outside of GVfs, but that would
result in considerable special-casing, and the author wouldn't gain
intimate knowledge of GIO.)
There is no perceived need to keep old collections when opening
new ones, so we simply change and reload the contents when needed.
Similarly, there is no intention to make the VFS writeable.
The process-locality of this and other URI schemes has proven to be
rather annoying when passing files to other applications,
however most of the resulting complexity appears to be essential
rather than accidental.
Note that the GTK+ file chooser widget is retarded, and doesn't
recognize URIs that lack the authority part in the location bar.
Windows and Linux applications are more likely to not bother,
and their desktop environments don't place windows right in the corner,
which is what happens with GTK+/macOS.
It would also be possible to handle this through press/motion/release
event handlers, though GtkGestureDrag is more convenient for hacking in
support for dragging to widgets not supporting GtkScrollable (yet).
There may be some undesired interactions lurking, besides the jarring
movements when dragging native GdkWindows (these are a pain).
The late, global gtk_widget_show_all() made it always start visible,
in particular when the program was launched directly on an image file,
and not in browsing mode.
