[FEATURE] Render icon with button view helper

Fluid is currently lacking a view helper to render a skinned icon.
By making the button icon view helper accept an empty URI as an
argument, we let it render an unlinked icon, rather than adding
a new view helper with very similar code.

This patch also cleans up the class phpDoc block, which
contained several errors.

Resolves: #51305
Releases: 6.2
Change-Id: I74b1dc3346e20c9094313a2da451bcb6c1f60f29
Reviewed-on: https://review.typo3.org/23281
Reviewed-by: Frans Saris
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind