[FEATURE] Refactor ElementInfo / show_item

Splits up functionality to represent sections.

Cleanup for html base: h1/paddings/p.

First step for MVC structuring; Render = View.

Brings visual improvements:
- human readable headline: table+title
- increase preview image dimensions
- download link to reference absolute file
- reference list table styling

Implementing folders' info is not in scope.

Change-Id: Ia411404a99df2d07dcce2d959ae324ec39966e04
Resolves: #49164
Releases: 6.2
Reviewed-on: https://review.typo3.org/21469
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn