[TASK] Move backend module icons to respective location

In TYPO3 CMS world each extension contains multiple icons:

+ "extension icons" (ext_icon.gif) to identify the whole extension (EM/TER)
+ "module icon" as Start button per backend module (module menu)
+ "record icons" in recordlist as clickmenu link
+ "action icons" like save, close or info (toolbar)

Currently all icons look the same in the backend (16x16 pixels, same theme).
Also many icon files are used multiple ways (e.g. ext_icon.gif as module icon).

In order to be able to differentiate icons better by their primary function
this patch introduces a common folder as defined location for backend
module icon files and demontrastes a naming schema "module-name.xxx".

Also this patch reduces cohesion and coupling of backend extensions since
extensions now bring their own icons in a predictable fashion.

This supports alternative backends that do not rely on t3skin at all.

Resolves: #61200
Releases: 6.3
Change-Id: I035b7777f0c59546ec641dbb59165a98fd887d44
Reviewed-on: http://review.typo3.org/32511
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>