gdk: Add GdkDragCancelReason enum as argument to GdkDragContext::cancel

gdk: Add GdkDragCancelReason enum as argument to GdkDragContext::cancel

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