Semantic labels on main nav bar

[gnprice]

This was found in #535 (comment) when investigating the app's behavior with a screen reader:

The home page bottom tabs do not have labels. The caption for each of them is just: "button, double tap to activate".

We should fix this.

This might get fixed as part of the same change as:

• Label icons on main nav bar #1857

[gnprice]

@MritunjayTiwari14, would you check whether your #1879 resolves this issue, and if so then mark it as doing so?

[MritunjayTiwari14]

Yes @gnprice , it can also fix it. In most app i checked out like Google and Whatsapp using TalkBack On. I noticed that both of them uses <navbar_item_name> Tab as a convention to announce the different bottom navbar items. So i am implementing the same in Zulip-flutter.