[FEATURE] Allow additional paths for extension icons

Until now, the extension icon had to be located under ext_icon.png,
ext_icon.svg, ext_icon.gif within the extension directory.

In order to prepare a way to allow only files under Resources/Public/
to be public, the icon can now also be put under
Resources/Public/Icons/Extension.png or .svg

Several paths where hard-coded extension icon paths are replaced
with the getExtensionIcon() API in the Core.

Resolves: #77349
Releases: master
Change-Id: Ib273f347498d412530ca256a083d896d168f65fa
Reviewed-on: https://review.typo3.org/49306


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>