Update install/uninstall buttons

[mheuzey]

When installing an extension, the 'Install' button doesn't change to let me know that it's installing, and there's no progress bar. It would be nice if the text could change to 'Installing' like the regular extension marketplace. A progress bar could also work, but the first option is more familiar.

It would also be nice if the 'Uninstall' button could change to a 'Reload Required' and/or an 'Install' button as well. Right now it continues to say 'Uninstall' after it completes.

[joelspadin-garmin]

The main reason I haven't done that is because syncing state between the extension and the web view is difficult (or if there's a simple way, I don't know what it is), and I didn't consider the effort required to be worth the time it would take to do. PRs are welcome though!