[BUGFIX] wrong set layer in page module during drag and drop

During drag and drop the dragged content element was
overlayed from the next content scope (headline element).

The "z-index" must be fixed here. Just added a hover effect
for the whole draggable content element. The correct value
for the "z-index" may between the z-index values of "topbar"
and "toolbar" (under "structure" stylesheets).

Resolves: #58413
Releases: 6.2, 6.1
Change-Id: I8fa83cec793b5deb7dbbc13553a043b706510904
Reviewed-on: https://review.typo3.org/29824
Reviewed-by: Sascha Wilking
Tested-by: Sascha Wilking
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring