add show/shown hide/hidden events to dropdown + update dropdown links for testing + catch a few missed namespace things

change dropdown strategy to use an overlay - fixes mobile click anywhere + allows for firefox middle click

* Fix as documented here: https://github.com/twitter/bootstrap/issues/4756#issuecomment-9952781
* Stops propagation for touchstart for dropdowns

Also fixes dropdowns hiding behavior for case when two or more dropdowns
with [data-toggle=dropdown] exist on the page

only stop prop on form – as people seem to be relying on dropdown hiding when clicking internal elements