Replace layout builder modal by layout builder iframe modal

Still need to implement an upgrade path, but would like to have a first review and impressions first.

Already a bunch of stuff that is working a lot cleaner, but still some issues remaining:

• When adding a block through layout builder, after adding the block, the page opens up in the layout builder iframe
• Some css improvements that where done now should be redone with the current setup
• Might be good to go over it with an ux expert to see what's still advised

Thanks a lot Brent for your testing and feedback.