Cursor over menu items should be a pointer

[lkovaleva]

I noticed that paper-dropdown and paper-menu both show a pointer cursor when hovering over their menu items, however paper-menu-button shows regular cursor over white-space and text cursor over menu item words. I believe these should be consistent and have a pointer everywhere.

[lkovaleva]

Behavior can be seen on the demo page: https://elements.polymer-project.org/elements/paper-menu-button?view=demo:demo/index.html&active=paper-menu-button