feat(select): Add enhanced select variant (#3949)

Adds the leading icon variant to the select.
Adds the helper text to the select.
Adds the rotating dropdown icon to the select.
Adds the enhanced menu version of the select.

BREAKING CHANGE: Several adapter APIs were added to support the enhanced variant. The drop-down arrow is now its own element. The change event is now MDCSelect:change for all variants. See the README for full details.

demos/index.html

@@ -164,14 +164,6 @@
             </span>
           </a>
 
-          <a href="select.html" role="listitem" class="mdc-list-item">
-            <span class="demo-catalog-list-icon mdc-list-item__graphic"><img src="/images/ic_menu_24px.svg" /></span>
-            <span class="mdc-list-item__text">
-              <span class="mdc-list-item__primary-text">Select</span>
-              <span class="mdc-list-item__secondary-text">Popover selection menus</span>
-            </span>
-          </a>
-
           <a href="slider.html" role="listitem" class="mdc-list-item">
             <span class="demo-catalog-list-icon mdc-list-item__graphic"><img src="/images/slider.svg" /></span>
             <span class="mdc-list-item__text">