Improve calculator's tooltip accessibility.

Add possibility to navigate through the hints via arrow keys.

**Keyboard Support**:
If focus is on the hint button:
  * Enter: Open or close hint popup. Select last focused hint item if opening.
  * Space: Open or close hint popup. Select last focused hint item if opening.

If focus is on a hint item:
  * Left arrow: Select previous hint item.
  * Up arrow: Select previous hint item.
  * Right arrow: Select next hint item.
  * Down arrow: Select next hint item.

**Goal** ([composite](http://www.w3.org/WAI/PF/aria/roles#composite)):
>Authors SHOULD ensure that a composite widget exist as a single navigation stop within the larger navigation system of the web page. Once the composite widget has focus, authors SHOULD provide a separate navigation mechanism for users to navigate to elements that are descendants or owned children of the composite element.


Related links: 
- [Menubar: ARIA](http://oaa-accessibility.org/example/26/)
- [aria-activedescendant](http://www.w3.org/WAI/PF/aria/states_and_properties#aria-activedescendant)
- [composite](http://www.w3.org/WAI/PF/aria/roles#composite)

@jmclaus , @auraz , @frrrances please review.