This is the remaining changeset of my earlier contributions that extended the oxygen-demo application with widget style and colour scheme pickers, aiming to turn it into a generic look-and-feel explorer.
There currently is no "official" cheap way to preview how checked menu items are rendered as a function of installed widget style.
This patch explores a straightforward way of adding that missing functionality via the most exhaustive look-and-feel preview utility I am aware of that is cheap in resource requirements: Oxygen's demo app.
This demo already has menus in the MDI preview. The patch makes the layout items checkable and puts them in a QActionGroup so their mutually exclusive nature is taken into account.
An earlier version added a menu action to toggle the layout direction (mostly to provide a non-exclusive checkable item) and made sure to initialise the layout direction checkbox as a function of the startup direction. The menu item was removed on request but I decided to leave the checkbox initialisation .