Move windows stacking code

The code related to windows' stacking order is moved from the class Workspace and the file layers.cpp to the win folder/namespace. A new Qt signal is introduced and some signal-slot connections are performed.

Splitting

Here we report the old names; the new ones are snake-cased.

win/stacking.h

• ensureStackingOrderInList, keepTransientAbove, keepDeletedTransientAbove

win/layers.h (new header)

• raiseOrLowerClient, raise_window, lower_window, restack, restackClientUnderActive, topClientOnDesktop, findDesktop

win/x11/control.h

• lowerClientWithinApplication, raiseClientWithinApplication, lowerClientRequest, raiseClientRequest

win/x11/space.h (new header)

• restoreSessionStackingOrder, stackScreenEdgesUnderOverrideRedirect

utils.h

• BlockStackingUpdate

New objects

win::stacking_order

• Signals: changed(), unlocked() (new)
• Attributes: stacking_order, unconstrained_stacking_order
• Methods: blockStackingOrderUpdate, updateStackingOrder, getConstrainedStackingOrder, forceRestacking, addManualOverlay, removeManualOverlay, propagateClients

win::x11::stacking_tree

• xStackingOrder, updateXStackingOrder, markXStackingOrderAsDirty

Functions removed

updateClientLayer, lowerClientRequest(Toplevel*)

Closes #152 (closed)