nemo-file.c: Allow files with backend-provided previews to be
thumbnailed, even if their filesystem forbids it.
Nemo's thumbnailer now returns the provided preview icon if one
exists. Actual thumbnail visibility is still controlled by the
user like normal.
This will not guarantee thumbnails, if the device doesn't have
any, but the couple Android devices I tried did have them.
ref:
https://github.com/linuxmint/nemo/issues/3055
https://github.com/linuxmint/nemo/issues/3458
https://github.com/linuxmint/nemo/issues/3291
https://gitlab.gnome.org/GNOME/nautilus/-/issues/1921
