[BUGFIX] Add button styling to clipInsert button

The button for inserting items from clipboard to a multiselect field has
a proper styling now.

Change-Id: I2360507fb0daedc57e0e1b3a5442af790529039e
Resolves: #74813
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/47172


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Gianluigi Martino <gmartino27@gmail.com>
Reviewed-by: Daniel Maier <dani-maier@gmx.de>
Tested-by: Daniel Maier <dani-maier@gmx.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>