[BUGFIX] LiveSearchToolbarItem not initialized correctly

Initialising the LiveSearchToolbarItem class with two leading 
backslashes \\ prevents XClasses of the LiveSearchToolbarItem
to be found by the GeneralUtility:makeInstance() function.

Resolves: #59959
Releases: 6.3, 6.2
Change-Id: Iab6bb31379090fa5770ed20f8fa8113e61071fe0
Reviewed-on: https://review.typo3.org/31475
Reviewed-by: Markus Klein
Tested-by: Markus Klein