Modal scrollbar handling problem when implementing stacked modals

Created by: cervengoc

Stacked modals can be easily implemented with a few lines of custom code for z-indexes and so (as mentioned in the docs).

However, there is a usability issue in the core code. Consider the following:

1. Body is overflowing vertically.
2. A first modal is shown, so scrollbar disappears, and some padding is applied to the body.
3. A second modal is shown, and paddig is applied again, for no reason.

This could be easily fixed if there was a piece of code and maybe a global helper variable to check if a modal is already open or something like that.