feat Implemented keyboard navigation for the MultiAutoComplete widget

In this PR comes:

- implementation
- tests
- some refactoring, especially to reflect some design changes

According to the specifications, it is now possible to use the left and right arrow keys to navigate throughout the whole widget, including currently inserted options. It is also possible to use the tab key to jump form the inserted options list to the text input field, and then from this field to the next focusable element in the page (outside the widget).