Allow custom renderer for group header (or tabs container)

[borglin]

Is your feature request related to a problem? Please describe.
Users with many tabs open find it hard to find the tabs and distinguish them from others, and they take up too much space. We'd like to group the tabs similar to what you can do in Chrome.

Describe the solution you'd like
Being able to specify the headerComponent for a group, in addition to left-, right- & prefixHeaderActionsComponent, like you can with a tabComponent but instead replacing the enitire content of .tabs-and-actions-container.

Describe alternatives you've considered
It could probably be solved by building more complex individual tabComponent that can render differently depending on how they are being grouped.

Additional context
This could also allow different implementations for handling tabs overflowing the header width.