Popover's boundary prop does not accept Element objects

Package: @blueprintjs/core@3.15.1

Issue: The boundary field of IPopoverSharedProps (defined here) does not accept values of type Element.

Description: This field (boundary) is only used as the boundariesElement field in flip and preventOverflow objects in Popper modifiers, as seen here. Both of these fields in Popper (defined here and here) accept objects of type Boundary | Element, but the boundary field defined in Blueprint only accepts Boundary.

Possible Solution: Change the type here from PopperBoundary to PopperBoundary | Element.