[BUGFIX] Enable BE search for multiple mountpoints

Backend search now allows search from all mounts of an editor.
If the root page is selected it will search in all mounts of the
backend user.

Change-Id: Ic462725bb4cba8100c5eeca5f7f47b5711ab2869
Resolves: #35073
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/24886
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer