[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.

Although initially planned for 6.2 only, this change
will go into 6.0 and 6.1 also to fix the regression
in #51767

Resolves: #51305
Resolves: #51767
Releases: 6.0, 6.1
Change-Id: I74b1dc3346e20c9094313a2da451bcb6c1f60f29
Reviewed-on: https://review.typo3.org/23651
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel