The shared dropdown component is made more-configurable, so it can be
reused in more contexts.
* The down-arrow is an flag that can be enabled on the dropdown, instead
of added (and styled) at the use site.
* Items no longer need to be links to new URLs. They can be buttons with
declared handlers.
* The dropdown trigger can be a link or a button, as configured by the
link flag.
Change-Id: Ia9db3b66bf30bc56bf2ab4ce1f2292f51ab4754d
bda4683444