Fall back to STANDARD_FAST_CONTENT_TYPE if STANDARD_CONTENT_TYPE is not set

[JBergles]

Goal

I was having issues with opening files/directories on gvfs remote mounts (ssh and smb). Whenever I'd try to open files on a remote mount, I'd get the The given locations contain no images. error overlay.

After a bit of digging, I was able to find that gvfs is no longer always providing standard::content-type for files.

This branch updates Viewnior to fall back to standard::fast-content-type if standard::content-type is not available

References

• https://gitlab.gnome.org/GNOME/gvfs/-/merge_requests/68
• https://gitlab.gnome.org/GNOME/glib/-/commit/5e1af07733e1133f60045c4a2164e2af40d3de7d