aad3135e4c
We should conform to a minimal set of reasons for the gtk side to emit a better GtkDragResult than GTK_DRAG_RESULT_ERROR. This fixes the notebook tab DnD feature, where we rely on GTK_DRAG_RESULT_NO_TARGET. In the wayland side, unfortunately we can't honor either NO_TARGET nor USER_CANCELLED, we don't know of the latter, so we could return false positives on the former. https://bugzilla.gnome.org/show_bug.cgi?id=761954
