[tabs] Support nested Tabs

[Dassderdie]

This is about the Tabs-Component.

We would like to use nested tabs like this:

<tabset>
    <tab heading="Basic title">Basic content</tab>
    <tab heading="Basic Title 1">Basic content 1</tab>
    <tab heading="Basic Title 2">Basic content 2
        <br>
       <tabset>
            <tab heading="Basic title">Basic content</tab>
            <tab heading="Basic Title 1">Basic content 1</tab>
            <tab heading="Basic Title 2">Basic content 2</tab>
      </tabset>
    </tab>
</tabset>

We are aware that nested tabs are not great ux-wise, but we need this for a part that we generate recursively and see no better option for this.

I'm not sure wether this is a feature request or a bug report for the docs, because the documentation states that nested tabs are not supported, but in a brief test the code above seemed to work as expected.