Improve style and behavior of widget highlighting

Some improvements prompted by @shaunandrews:

 - What color/style should be used to indicate a highlighted widget? 
 - Should there be a different style for a widget in the preview than a widget customizer control?
 - Should the highlight be persistent instead of going away after 500ms?
 - How/when should widgets in the preview window (and in the customizer) be scrolled to if they are not in view when being highlighted? (There needs to be a smooth animation.)

Builds upon #13.