[BUGFIX] t3skin calls addIconSprite for each lang

The t3skin extension adds icon sprites for each language. And that's
great, but not so great that it calls addIconSprite for each language.

Instead the iconArray can be built up and submitted once to
addIconSprite.

This saves 248 calls to addIconSprite which calls array_merge etc.

This saves ~ 22 ms for each request.

Change-Id: I0fdc09de46899e4160f907aefd8b3b3b596a2df3
Resolves: #53918
Releases: 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/25684
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe