Layout menu feature PR: #770

Open items:

• Should we do some validation for 'args' depending on method or just let Plotly[method] error out?
• Should add more global settings:
  • width
  • height (instead of being inferred by the label dimensions)
  • title (above header)
  • header borderwidth
  • active / hover bgcolor
  • gap between buttons
  • gap between header & buttons
• We should probably make dropdown items scrollable
• Should the dropdown arrow direction depend on direction see Add button type to updatemenus #974 (comment)