[TASK] Improve Switch-User concept in ext:beuser

The "Switch-User" functionality can be improved:
* It should be possible to switch to other admin user,
  just not the current user
* The title of the link is reintroduced
* The order is as in previous TYPO3 version,
  switch-back-mode is the 2nd one

Resolves: #42502
Releases: 6.0

Change-Id: I7d2a32d5682731798a0671715b2bbfac9a6eee77
Reviewed-on: http://review.typo3.org/16024
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel