Don't allow drops in unmounted volumes
Don't allow drops in unmounted volumes

Unmounted volumes don't have a URI yet, so we can use that to distinguish them
from within the DnD code.

Signed-off-by: Federico Mena Quintero <4999915a961edfd7686112c2935288e1266eae14@gnome.org>

Sidnioulz/SandboxGtk,jigpu/gtk,jessevdk/gtk,ebassi/gtk,ebassi/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,bratsche/gtk-,alexlarsson/gtk,grubersjoe/adwaita,ahodesuka/gtk,bratsche/gtk-,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,ahodesuka/gtk,ebassi/gtk,Lyude/gtk-,jigpu/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,chergert/gtk,chergert/gtk,alexlarsson/gtk,alexlarsson/gtk,ebassi/gtk,alexlarsson/gtk,jigpu/gtk,ahodesuka/gtk,Distrotech/gtk2,jadahl/gtk,chergert/gtk,chergert/gtk,ahodesuka/gtk,jessevdk/gtk,bratsche/gtk-,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,bratsche/gtk-,Distrotech/gtk2,alexlarsson/gtk,Lyude/gtk-,Adamovskiy/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,grubersjoe/adwaita,Distrotech/gtk2,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,jigpu/gtk,ahodesuka/gtk,ahodesuka/gtk,bratsche/gtk-,ahodesuka/gtk,Adamovskiy/gtk,jessevdk/gtk,chergert/gtk,jadahl/gtk,Distrotech/gtk2,ebassi/gtk,msteinert/gtk,davidgumberg/gtk,jessevdk/gtk,jigpu/gtk,msteinert/gtk,Lyude/gtk-,Adamovskiy/gtk,davidgumberg/gtk,davidgumberg/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,ebassi/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Lyude/gtk-,jadahl/gtk,davidgumberg/gtk,jessevdk/gtk,alexlarsson/gtk,davidgumberg/gtk,ahodesuka/gtk,Lyude/gtk-,grubersjoe/adwaita,Lyude/gtk-,jigpu/gtk,msteinert/gtk,grubersjoe/adwaita,bratsche/gtk-,jadahl/gtk,grubersjoe/adwaita,Distrotech/gtk2,chergert/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,Adamovskiy/gtk,chergert/gtk,msteinert/gtk,alexlarsson/gtk,Distrotech/gtk2,msteinert/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-