Reintroduce bespin fix for nasty splitters.

Hi, you probably forgot to add this cause you have included splitterproxy.h in the repo.
You also probably know what I mean by nasty splitter behavior.
(mouse pointer stays grabbed even though I just passed over the splitter)

That behavior is apparently not dolphin's fault.
My own qt5 widgets based app has a dock just like dolphin and splitter misbehaves just like dolphin's.

I just copied missing lines from bespin.
I considered changing splitterproxy timer to just 30, but I believe it's just placebo thinking it's better.

I wonder where the bug really is, cause qtcurve doesn't need such hack for splitters.