Add xdg-toplevel-drag protocol

This protocol allows applications to request that a window is moved
at the same time as a drag operation - effectively dragging windows.
With this features such as detaching a tab from a window and reattaching
it, dragging tabs between windows or (un)dockable tool windows can
be implemented.
Based on the previously proposed extended drag protocol but trimmed
down.

Signed-off-by: David Redondo <kde@david-redondo.de>