[BUGFIX] Wrong path generated for icon in NewContentElementController

[BUGFIX] Wrong path generated for icon in NewContentElementController

When using EXT:myext/icon.png in the new content element wizard,
a wrong path is generated. The current solution only works for core
extension. Fix this by creating a correct relative path.
Allow usage of absolute paths too.

Resolves: #68575
Releases: master
Change-Id: Ideccb89da85fc81722e880f386514e56feb88069
Reviewed-on: http://review.typo3.org/42044
Reviewed-by: Georg Tiefenbrunn <georg.tiefenbrunn@gmail.com>
Tested-by: Georg Tiefenbrunn <georg.tiefenbrunn@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>