[TASK] Streamline styling of RTE context menu

The visual appearance of the context menu of the RTE is streamlined with
the non-ExtJS context menus. The "scroller" areas are now larger and
easier to hit.
The shadow which is behind the context menu has been removed.

Resolves: #75992
Releases: master, 7.6
Change-Id: I3b360ce3fa2e9cae68d796e0a612c8f443c0141d
Reviewed-on: https://review.typo3.org/47978


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>