[FEATURE] API for Bootstrap Modals

This patch introduces a new BE API for modals.
We introduce only a confirm method, because
the modal should be used as few as possible and
only for small information or confirm dialogs.

Resolves: #63729
Releases: master
Change-Id: Ie2ed6742fbebbc982afabb4eff01045fb2318492
Reviewed-on: http://review.typo3.org/35313
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>