KDevelop uses that as well, in addition to a "noraml" element comletion, you just need to let it know where to look for the information.
But any class struct etc that you declare in your project is automatically known, and will pop up word copletion when using these objects with no extra twicking.
Theoretical yes... BUT this feature in KDevelop is on very low level, in my opinion of course. If I want to see hints of qt classes it only shows few of them. Maybe there smth wrong with configuration and I don't know what but I really spend lot of time to make it like I want it...

About my problem with slickedit... I resolved the problem with word completion. I can only tell this IDE is GREAT