I have considered making a window manager a few times, but have always stumbled since you have no control over Z layer for any components (the only way to affect Z is for a single component that can be pulled to the front via setting the currect focus, which is far from useful in this context). I noticed CT_CONTROLS_GROUP mentioned but with no examples or documentation for that control type, I couldn't use it. The total crapness of the armoury probably didn't encourage many people to delve into it, but if you've found at least the basics of how that control works I think a lot of people, including myself, would be grateful if you could explain its use. Would be nice if you could save us all searching for the examples and doing all the tests you have presumably done on this already.
Both i0n0s and I (and probably others) have implemented systems to manage dragging groups of controls in order to make pseudo-windows. i0n0s's system is used in RTE but has already been released as a separate script, but mine is only used within SPON Map.