refactor(combo): Use key-bindings controller

This commit migrates the combo component to use
the general key-bindings controller from the library.
Aside from unifying the implementation across components
this also improves several behaviours of the combo:

* Keyboard events inside the combo are no longer "swallowed".
* Tab navigation is more natural, i.e. there are no longer 2 tab stops
per combo component.

Updated the combo test suite to accomodate for the
changes.

Author: rkaraivanov