[WIP] preview bar

This pull request is for implementing the previewbar without frames.

See  #7237.

> This is not a real Front end edit mode, but allows much more as the current status and saves many clicks ;). I have already made a first draft. If you want to see it in action, tell me. I can set up a Contao with some dump content to play with it.

- [x] remove the frame
- [x] add a topbar like the dev tools in the bottom
- [ ] style the top bar -> Anybody out there who can do this? ;)
- [ ] close button for the bar, or can I remove it?
- [ ] page edit link (open in modal)
- [ ] article edit links (open in modal)
- [ ] content element edit links (open in modal)
- [ ] layout edit link (open in modal)
- [ ] layout module edit links (open in modal)
- [ ] contao conform code style

So let me know what else is missing ;).