Add option to not close Modal when the user clicks outside of it

[cvrebert]

Currently, a Modal closes whenever the user clicks outside of the Modal.
This is reasonable in many cases, but in other cases, it is necessary to force the user to make an explicit choice/decision (rather than just the implicit choice of exiting the Modal) before letting them move beyond the current Modal. It would therefore be useful for Modals to have a "don't close the Modal when clicking outside of it" option (which would default to the current behavior).