Improve design of layout content nodes in layout mode

[manuelmeister]

I aligned the design of the layout content nodes in the layout mode to material design.

Implements/improves content node design of #4182

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[usu]

Like it. A few comments, some of them not directly related to the improvements from this PR, but it's a good opportunity to throw them in 😄

• The title of each section (e.g. "Print above main content") is easily read over. I would have emphasized this more (different color? larger text?).

• For complex layouts, there are a lot of "add buttons". Would it help, if we add a little hint, what/where people are adding stuff? (e.g. "Add (above main content)" or "Add (column 1)"). The second part could also be much smaller to avoid too large buttons.

• The last "add" button is always a bit confusing, because in paper size (or smaller screens) there are 2 add buttons immediately after each other and I believe it could be non-trivial for users to understand the difference. Would it be an idea, that we restrict the last button to layouts only (column layout or responsive layout) and we call the button "Add layout".

[manuelmeister]

@usu thanks for the review.

I tried to incorporate most of your feedback.

I would not restrict the last (most outer) button to layouts only, as it is a perfect valid use case to just have normal content nodes inside our root columnlayout.

[BacLuc]

Could not look through the code and not test it thoroughly.
Looks good on the first glance

[usu]

Also, I noticed a case where the column resize buttons of the column layout disappear when I shrink the window size (happens at the same time as the sidebar collapses). Not sure, though, if this is related to this PR.
Couldn't do a recording (unfortunately my screen recording tool is crashing constantly...). Hence only 2 screenshots below.

[manuelmeister]

@usu fixed the hidden resizer button and the disabled inputs.

[manuelmeister]

I think I would merge #4264 first and then test this PR again before we merge it.