Modal Backdrop (nearly) never fits

Created by: DaSchTour

Because of the change with modal backdrop and the new function from #15106 (closed) there are a lot of problems with the modal backdrop now.

1. It's a major break in backward compatibility. I have to rewrite the complete handling of modals to ensure that with every action inside the modal the backdrop is updated
2. Even bootstraps own functions don't use the modal handling. So with every collapsable panel inside a modal I will have problems with the backdrop.
3. Why the hell was this changed in 3.3.0? From my point of view the modals worked perfectly before 3.3.0!
4. Is there any solution to attach a "size changes" handler to the modal that updates the modal without having to manually call the the update handler?