Add focus state rings instead of hover to buttons

[elioqoshi]

Currently buttons focus states use the hover state for focus state (as Google does the same with M3, I assume Yaru took the same). This creates accessibility issues when navigating buttons via keyboard where it's not clear which button is focused.

A good solution for this would be to add focus rings for the focus state of these buttons, e.g:

The focus rings could use the same color as the button (or same color theme at least) so it's consistent with the button styling. Please note that this focus state would be basically on visible when navigating via keyboard.

Some references: https://dockyard.com/blog/2020/04/28/designing-for-accessibility-focus-states

[Feichtmeier]

Now that all yaru libs are merged this is either a theme issue or a widget issue depending on if we want to create or fully own buttons or if we want to get this into material.dart on flutter/flutter