servoshell: add a spinner that indicates when the page is loading

[delan]

When the minibrowser is enabled, add an egui spinner to the location bar when the page is loading.

Watch for EmbedderMsg::LoadStart, EmbedderMsg::HeadParsed, and EmbedderMsg::LoadComplete events, and keep track of whether the page is currently loading in Minibrowser.

This may be a good newcomer issue once #31688 is fixed.

[frereit]

Darn, I forgot to leave a comment here that I started work on this, sorry. I hope this didn't lead to doubled work but I've done some work in #31713. If anyone else also worked on this, maybe let me know and we can discuss the issue in more detail 😄