Add a loading/reloading indicator to the UI

[kelsey-taylor]

Describe the issue

If you have a lot of styles that rebuild each time you make updates when working locally, it can be hard to tell when Maperture is rendering the updated styles. When used in concert with the build system, the only way to tell that styles are fully reloaded (afaik) is to keep an eye on the terminal.

It would be helpful to have a visual indicator (like we do in Chartographer) to indicate that something is rebuilding in the background.