[FEATURE] Add a "virtual root" to pagetree (for mountpoints)

If there is no "root" of pages (page 0) shown, add a
"virtual root" that functions as a root for all mountpoints
below it.

Change-Id: Iab25790db1ac5de4ab333d605163355eb1c62b91
Resolves: #33852
Releases: 4.7
Reviewed-on: http://review.typo3.org/8977
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter