[BUGFIX] Additional margin on top of ExtJS context menus

Resolves: #56950
Releases: 6.2
Change-Id: I936f75be45447111083b9f5cae2e88d3fac1d2c5
Reviewed-on: https://review.typo3.org/28435
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny