Use the plugin ID as class in toolbar since titles might change

Problem/Motivation

#2256497: [meta] Menu Links - New Plan for the Homestretch is making some menu links plugins.
#2301317: MenuLinkNG part4: Conversion has a @todo in toolbar_menu_navigation_links()

Proposed resolution

Use the plugin ID as class in toolbar since titles might change.

Remaining tasks

User interface changes

No.
But might effect themers.

API changes

No.