Instead of an obscure Gtk setting, allow a configurable value for the
panel/clutter DND drag threshold.

Default to 8 (which is the gtk default)
Move dnd threshold to Mouse panel, change GTK threshold slider to a spinner
to allow more granular control of both DnD settings
Remove orphan label
Add "advanced" muffin edge tile and detach threshold settings to window panel
Merge pull request #1751 from mtwebster/dnd-threshold

Provide more control for DnD features of Cinnamon and Muffin