[BUGFIX] Make be.buttons.icon-ViewHelper extensible

Many of the standard-ViewHelpers (tag-based) at least support
additionalAttributes. Introduce this for be.buttons.icon as well
to allow for attributes like onclick or target. Those arguments
will be assigned to the surrounding a-tag if an uri is specified.

Change-Id: I0b8ef12842a818ed5d49437a003287aff9b08f13
Resolves: #53538
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/25293
Reviewed-by: Eric Chavaillaz
Tested-by: Eric Chavaillaz
Reviewed-by: Pascal Dürsteler
Tested-by: Pascal Dürsteler
Reviewed-by: Markus Klein
Tested-by: Markus Klein