Finish off mobile toolbar functionality and propose as patch for Drupal 8

This includes, but is not limited to:

- Incorporate Shortcuts into the tray
- Some sort of hook for page actions (edit module actions) *and* the ability for other modules to add more actions *and* the ability for modules to override how the expansion works (dropdown/accordion/…)
- Type ahead menu item autocomplete
- Fine points of styling
- Turn the User actions in the Toolbar into a dropdown
- Testing, testing, testing on small devices

Work is happening over at #1137920: Fix toolbar on small screen sizes and redesign toolbar for desktop. Marking postponed in favour of that issue.