the mainwindowlayout should respect the sizePolicy/minsize/maxsize of the centralWidget and the dockwidgets

With a central widget and a dockwidget at right, if you set a fixed size for the centralWidget and set the sizePolicy to expanding to the dockwidget, mainwindowlayout doesn't enlarge the dockwidget, but still let the central widget to expand. This leads to a not very good result, an empty gray area fills in the gap between the small central widget and the dockwidget, which is annoying.

We expect:

• mainwindowlayout adjusts widgets size according to their size policy (like normal layouts).
• the widget with expanding policy to occupy the extra space. in this case central widget should only occupies the fixed size, and the dock widget should expand to occupy all extra space.
• empty gray area to fill the extra space if there is no other widget fitful.