grid-cell, name-cell: Allow displaying unthemed emblems

nautilus-dropbox and probably other such extensions use emblems to
indicate sync status.

If these icons are installed as unthemed icons, then we don't
display them at all. This is because the way we are checking that
an icon exists ignores the unthemed icons completely, which is
arguably a GTK bug (https://gitlab.gnome.org/GNOME/gtk/-/issues/5709)

While the GTK inconsistency is not addressed, let's use a workaround
to check more effectively whether an icon exits.

Fixes https://gitlab.gnome.org/GNOME/nautilus/-/issues/2789


(cherry picked from commit 032d7d00)