[FEATURE] Refresh the look of pagemodule

The patch simplifies and optimizes the readability of the page module
interface. It is based on the UX Team's concept of improving user
experience for editors:
https://typo3.org/article/structured-content-initiative-feedback-wanted

Included changes:

* refactored and optimised underlying CSS for future adaptions
* visually simplified content element boxes and their header buttons
* visually differenciated hidden content elements with better opacity
  and dotted border
* placed the "new content element" buttons centered in preparation
  for further concept implementations in dedicated patches
* added a new button for content element context menu in the content
  element header right button bar

This patch was initiated by the UX Team and implemented by Benjamin
Kott.

Resolves: #97306
Resolves: #95247
Related: #94629
Releases: main
Change-Id: I9b4e21a7fc68ebcb17d18f65bf00096f3a295cfb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74254
Tested-by: Benj...