Smarter opaque resize

Opaque resize on a splitter means that the widgets affected by the splitter position will be resized only after the user has finished moving the splitter.

It'd be nice if the widgets were updated also while the moving of the splitter is still in progress, but only if the mouse was not moved for a certain amount of time.

Imagine a user moving the splitter, pausing for a second with the mouse button still down and then getting presented widgets with updated sizes. If he likes the configuration, he can just release the mouse; if not, he continues moving the splitter.