[TASK] ToolbarItem registration and position

Toolbar items are now registered with its class name in array
$GLOBALS['TYPO3_CONF_VARS']['BE']['toolbarItems'].

The ToolbarItem interface adds a position method to determine
the order of items.

Resolves: #62959
Releases: master
Change-Id: Ie18d4b2148093f28c1f5387907912d8591797582
Reviewed-on: http://review.typo3.org/34149
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>