base.MDCRoot is now simpler in a number of ways. It now only contains
the minimum of information needed to start an MDC component: the
component class and the targeted root element to instantiate with.
Components now embed MDCRoot directly to automatically implement
vecty.{Un}Mounter.

API CHANGES
===========
Most of the common fields between components have been replaced by
functions in a new package, base/applyer. For example, add
applyer.CSSOnly() to a component's markup to disable Javascript/ripple.

As for the Ripple field, it has been replaced by a dedicated ripple
package and struct that implements vecty.Applyer as well.

Finally, this commit begins using a convention in all components where
the major elements in a component are struct fields of type
vecty.MarkupOrChild. Every component has a Root field like this, which
essentially replaces the Markup field. If Root is set to something other
than just Markup, it is considered a wholesale replacement of ...