Improve performance when saving sub-state

[MLFlexer]

If a user wants to save all workspaces + named windows, then the state for the workspaces would be saved which would include the windows. Then the windows would be saved. This essentially computes the window state twice. The same thing happens for tabs.

This redundant computation can be removed by saving the sub-state (windows in the example above), when computing the state of the parent (workspaces in the example).

This should be a "free lunch" if it is enabled via. Opts to the save functions.