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/23895
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer