Modal window buttons

As suggested in https://github.com/contao/core/issues/5985 here is a pullrequest to get the Close Button in one line with the tl_submit Buttons.

Moved the tl_submit buttons to the footer in the modal window …
- [x] Added Modal Window Footer again
- [x] Added template var to define an extra Footer Class
- [x] Added CSS to style the tl_submit Buttons to look like the Modal Window button
- [x] Added CSS to move the modal Window Button to the right place

For Testing I have over copied the minified files. (Not forget to purge the generated JS and CSS files)

Is it possible to get this in the 3.2 Release?