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Most of the common fields between components have been replaced by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As for the Ripple field, it has been replaced by a dedicated ripple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 the
built-in component element. Built-in root element markup is then applied
in an Apply() method of the component, which means you can use any
component as an Applyer on existing HTML. This type of customization is
a WIP.