[TASK] Style the "add irre item" button

This changes styles the buttons to add a new irre item like
the button to add images to a content element.

This patch also unifies button styles.

Resolves: #55539, #53218
Releases: 6.2
Change-Id: Ib07e51cfc6b6675671e1c6a2afd7bb0ebe379e1e
Reviewed-on: https://review.typo3.org/27221
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack