Fix loading screen for default bevy app setup

[TimJentzsch]

The loading screen with the default index.html we serve doesn't work for the default app setup, it isn't removed from the webpage.
It currently assumes that the canvas #bevy is used to create the app, but this needs to be configured explicitly in the Bevy app (as it was done in the bevy_quickstart template).

However, the default index.html should work with as little adjustments to the app as possible.

Therefore, we shouldn't assume a specific ID of the canvas. Instead, we can maybe observe when a new canvas element is spawned and use that event to remove the loading screen.

[BD103]

Closed by #144 :)