[BUGFIX] Fix dragging of elements in page module

When dragging content elements in the page module, the element gets
overlayed by TCEform elements. It loses its hover state, when dragged
outside the viewport or very rapidly.

* Set z-index higher than 4000
* Add a css class to stabilize active drag state
* Remove duplicated javascript code in page module

Resolves: #58473
Releases: 6.2, 6.1
Change-Id: I87577d30e5d6bb16f6999d466bef92da95b2add3
Reviewed-on: https://review.typo3.org/29837
Reviewed-by: Felix Oertel
Tested-by: Felix Oertel
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes
Reviewed-by: Wouter Wolters