Accessibility: active tab is differenciated only by color

[Dehelssey]

The active tab in the bottom nav bar is only differenciated from the others by its color. It goes against accessibility best practices

Active tab should be highlighted with an additional marker: either the icon goes from linear to filled, or a small indicator could be displayed underneath (a dot or a bar for instance)

[shuuji3]

Yes, once the nav button labels are implemented in #3119, we can switch icons to filled-style ones. While some icons don't have filled styles (like hashtag icon #), we can distinguish the active button by making labels bold / adding highlights.

[Dehelssey]

We can also have a background like on Material examples if we prefer.

I'm more into dots below the tab but that's purely subjective