Add support for showing containing folder

Fixes #120 (closed) and helps #54 (closed) (the "access a viewed image within Thunar" part).

(The name comes from Mint's xviewer)

The EXO_{CFLAGS,LIBS} is not cleaned up on its previous usage.

exo (>= 4.16) will fallback to xdg-mime when xfce4-mime-helper is missing so this should work everywhere I think.