Allow to globally customize DOM Nodes for Portal and Modals to support Micro-fontends scenarios

[jakobz]

When developing micro-frontend patterns, it's beneficial to isolate app parts into Shadow DOM. In this case, we need to allow to customize to which node we inject Portals (e.g. Dropdowns), and modal windows.

We can implement this as a react context (or reuse existing, e.g. LayoutContext), where we place these nodes.