Introduce transient constraints api in Workspace

Currently, the Workspace has no any api to constrain one window above
another. This results in having hacks such as keepDeletedTransientAbove()

This change introduces a basic api to constrain a given window above
another. It can be used for ensuring that transient windows are placed
above their parents. It also can be used for stacking the outline window
below the move-resize window.

Internal windows may also have transient parents. Because of that, this
change makes the workspace add internal clients to the stacking order by
default. The good thing about it is that it allows us unify some input
related code for "external" windows and internal windows.