[TYPO3-core] RFC #10651: Add <abbr> tag to hints SL, GL, SGL in EM

[Steffen Gebert]

Francois Suter wrote:
> So here's an update to Steffen's patch that adds a "and System
> extension" to the abbreviation's text when an extension is installed as
> "SGL". This patch also includes corrections related to CGLs and better
> comments in the code.

Thanks for your overwork, Francois.

My first versions would have outputted "local extension takes precedence 
over global extension takes precedence over system extension".
Later I decided to make the patch more simple and removed the hint for the 
third installation place.

P.S: I would name $overriddenExtensions $overriddenExtension instead, as 
it's only one extension.

Regards
Steffen