v5: export utilities such as EventHandler and SelectorEngine

Created by: mistic100

I maintain https://github.com/mistic100/Bootstrap-Confirmation which is an abstraction above Popover to add contextual buttons.

While removing jQuery dependency I find myself having to copy at lot of utility code from Boostrap such as EventHandler, SelectorEngine etc.

It would be much useful for plugins developpers to be able to import such utilities from Boostrap itself instead of duplicating code.