Generalized slice layout engine

[dalyIsaac]

This PR adds a generalized SliceLayoutEngine, via the SliceLayoutPlugin.

SliceLayoutEngine is a layout engine that internally stores an ordered list of IWindows. The monitor is divided into a number of IAreas. Each IArea corresponds to a "slice" of the IWindow list.

    context.WorkspaceManager.CreateLayoutEngines = () => new CreateLeafLayoutEngine[]
    {
        (id) => new SliceLayoutEngine(
            context,
            sliceLayoutPlugin,
            id,
            new ParentArea(
                isRow: true, 
                (0.5, new OverflowArea()), 
                (0.5, new SliceArea(order: 0, maxChildren: 2))
            )
        ) { Name = "Overflow on left" },

        (id) => new SliceLayoutEngine(
            context,
            sliceLayoutPlugin,
            id,
            new ParentArea(
                isRow: true, 
                (0.5, new SliceArea(order: 0, maxChildren: 1)), 
                (0.25, new OverflowArea()), (0.25, new OverflowArea())
            )
        ) { Name = "Multiple overflows"},
        
        (id) => SliceLayouts.CreateMultiColumnLayout(context, sliceLayoutPlugin, id, 1, 2, 0),
        (id) => SliceLayouts.CreatePrimaryStackLayout(context, sliceLayoutPlugin, id)
    };

There are three types of IAreas:

• ParentArea: An area that can have any IArea implementation as a child
• SliceArea: An ordered area that can have any IWindow as a child. There can be multiple SliceAreas in a SliceLayoutEngine, and they are ordered by the Order property/parameter.
• OverflowArea: An area that can have any infinite number of IWindows as a child. There can be only one OverflowArea in a SliceLayoutEngine - any additional OverflowAreas will be ignored. OverflowAreas implicitly are the last area in the layout engine, in comparison to all SliceAreas.

The SliceLayouts contains methods to create a few common layouts:

• primary/stack (master/stack)
• multi-column layout
• three-column layout, with the middle column being the primary