[REG]: When adding and showing a central widget in a QMainWindow then the layout does not respect the size policy of the central widget

When adding and showing a central widget in a QMainWindow then the layout does not respect the size policy of the central widget, before in 4.6.2 or earlier in the example it would take up a lot of space and resize the dock widget to its preferred size. Now in Qt 4.6.3 and later it will not take up as much as space as possible as the size policy permits.

Change 059be19781a22d2e41f22072152589857d0fabf9 is the one that broke this.