Add className from the model to Tab

[sorashi]

The className property on the model in type: "tab" wasn't actually applied to the final Tab.

Example layout:

const layoutModel = FlexLayout.Model.fromJson({
    borders: [],
    layout: {
        type: "row",
        weight: 100,
        children: [
            {
                type: "tabset",
                weight: 50,
                children: [
                    {
                        type: "tab",
                        name: "Tab",
                        component: "tab-component",
                        className: "customclass",
                    }
                ]
            }
        ]
    }
})

Result is now:

<div class="flexlayout__tab customclass" data-layout-path="/ts1/t0" style="left: 118px; top: 32px; width: 546px; height: 890px; position: absolute;">

[sorashi]

This is an updated version of #74 which is old and has conflicts