[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>

typo3/sysext/backend/Classes/Form/Element/AbstractFormElement.php

@@ -640,6 +640,7 @@ abstract class AbstractFormElement extends AbstractNode
                 $aOnClick .= 'return false;';
                 $icons['R'][] = '
 					<a href="#"
+						class="btn btn-default"
 						onclick="' . htmlspecialchars($aOnClick) . '"
 						title="' . htmlspecialchars(sprintf($languageService->sL('LLL:EXT:lang/locallang_core.xlf:labels.clipInsert_' . ($mode == 'db' ? 'db' : 'file')), count($clipElements))) . '">
 						' . $this->iconFactory->getIcon('actions-document-paste-into', Icon::SIZE_SMALL)->render() . '