feat(widget-markdown): add headings dropdown (#2879)

packages/netlify-cms-ui-default/src/Icon/images/_index.js

@@ -18,6 +18,7 @@ import iconGitlab from './gitlab.svg';
 import iconGrid from './grid.svg';
 import iconH1 from './h1.svg';
 import iconH2 from './h2.svg';
+import iconHOptions from './h-options.svg';
 import iconHome from './home.svg';
 import iconImage from './image.svg';
 import iconItalic from './italic.svg';
@@ -66,6 +67,7 @@ const images = {
   grid: iconGrid,
   h1: iconH1,
   h2: iconH2,
+  hOptions: iconHOptions,
   home: iconHome,
   image: iconImage,
   italic: iconItalic,

packages/netlify-cms-ui-default/src/Icon/images/h-options.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="22" height="20" viewBox="0 0 22 20"><path fill="#7A8291" d="M3 4h10v12H3V4zm2 0v5h6V4H5zm0 7v5h6v-5H5M17.874 16.91l-3.493-4.117h6.986l-3.493 4.117z"></path></svg>

packages/netlify-cms-ui-default/src/styles.js

@@ -312,11 +312,13 @@ const components = {
     display: flex;
     justify-content: space-between;
     align-items: center;
+    white-space: nowrap;
 
     &:last-of-type {
       border-bottom: 0;
     }
 
+    &.active,
     &:hover,
     &:active,
     &:focus {