This is a continuation/transfer of https://git.reviewboard.kde.org/r/128109/ (which still has a few screenshots)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, nor an easy/cheap way to compare the look of a given widget style under different colour schemes. I have been annoyed by the fact one has to (re)launch often heavy applications to assess the impact of changes to these look-and-feel aspects.
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. It also adds a "right to left" non-exclusive checkable item which is linked to the "Left to Right" checkbox in the demo frame.
For colour scheme exploration I adapted a thin wrapper around KColorSchemeManager written originally by Alexander Zhigalin for KDevelop;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. it is used to add a button with drop-down menu to the page widget's button box which makes it available in all viewsThe menu item was removed on request but I decided to leave the checkbox initialisation .