[FEATURE] Introduce System Information dropdown item

This patch introduces a new System Information dropdown item that
contains several system information. By default it displays:
* PHP version
* Database version
* Application context
* git revision (if TYPO3 version has "-dev" suffix)
* Operating system

The item list is extendable by the SystemInformationHookInterface.

Resolves: #65767
Releases: master
Change-Id: I2e7489f53198d44edea9995fe4cce49696a2f888
Reviewed-on: http://review.typo3.org/37881


Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>