[linearView] Fix dnd to dash when zoomed out

[linearView] Fix dnd to dash when zoomed out

While zoomed out, the workspaces view's drop target spans the entire
monitor to implement reactive screen edges. When a drop event is not
handled by the view, an attempt is made to pass it on down the stack,
but it doesn't work properly. Fix it by iterating the target's parents
as well.
Also improve the code which translates dnd coordinates to target
positions.

https://bugzilla.gnome.org/show_bug.cgi?id=619203