Add a new component to a container

Add a new component to an existing container.

We list the catalog components in the same place. Click on them to start the 'adding' state, click on a container to add the component to the container.
Clicking anywhere but on a container will cancel the 'adding' state. Styling of containers in 'adding' state still tbd.

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Use the existing solution: drag a component from the 'catalog' in the left side-panel into a container.

Possible implementation: only in this case, render overlay elements inside the app (as before) and drag into this overlay. That should make it possible to still use the Dragula library since all elements live in the same frame (dragging from outside an iframe into an iframe is not possible with Dragula).
--> WON'T WORK: we cannot scroll the overlay in the app anymore, so we cannot position it correctly in a scrolled page, nor can we scroll the page while dragging a new component around in search for a container to drop it in
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