Support XDGShell Positioning. This gives a client a lot more control
over where the popup will be placed as well as control over how to
handle constraints. i.e what to do if the popup doesn't fit.
trasientOffset was replaced with a method on the client as semantically
it's the role of the client to handle constraints.
Both slide and flip constraint adjustments are implemented. Resize
constraint adjustment will be handled in a future patch.
WlShell is handled by treating it as 1x1 sized anchor with slide