Modularize menubar mixins and add submenu indicator for top level items

Refactored menubar mixins so that less overrides are necessary from
normal button styles, and so that custom colored and custom sized menu
bars are possible.

Add new examples to test app for “drop down buttons” and “split
buttons” using MenuBar.

Change-Id: I2b0b76817d1c9119f82b2c2f4d7e5b95a9f05a5c