Feature request: Better support/demos for nested dropdown menus

[taranir]

I'm currently trying to use paper-menu-button to create nested dropdown menus, by nesting a paper-menu-button inside of the parent menu's paper-item element. Example menu: http://i.imgur.com/vDvDVTG.png
I can't use paper-submenu because I'd like to mimic the behavior of system menus (File, Edit, etc).

This somewhat works, but has many a11y issues (pressing enter key to select the submenu's trigger doesn't open the submenu, voiceover doesn't properly express that the submenu's trigger is a dropdown element, pressing esc to leave the submenu has inconsistent behavior) and I've had to make several hacky workarounds.

It would be great to have an official implementation of nested dropdown menus, with better a11y support, and a demo for nested dropdown menus on the Polymer website.