[BUGFIX] Unique IDs for pagetree root nodes

To make sure that every ID in the pagetree is unique in pagetrees
with a virtual root node (which is used for normal editors) the
page UIDs of the webmounts are set in the mountpoint property of
the tree nodes. This makes sure that all tree node ids are unique.

Resolves: #57163
Releases: 6.3, 6.2
Change-Id: Ia938a9bf4839524d60056d55c0186196dbd11289
Reviewed-on: https://review.typo3.org/31246
Reviewed-by: Markus Klein
Tested-by: Markus Klein