Semantics for loading screen/indicator

[gnprice]

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

The loading screen/loading indicator cannot get focused. There should at least be some way to tell if the page is loading.

We should fix this.

A required part of any PR to address this issue will be to try it out with a screen reader and confirm how it behaves. See the description of #535 for instructions.

[cybernova2]

Hi! I’d like to work on this issue.

I’ve added Semantics widgets to all loading indicators so they are focusable and announced by screen readers. VoiceOver/TalkBack now correctly reads “Loading…” when progress indicators appear.

I will open a pull request shortly.