Add data, datastart and dataend events

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There are events for when individual tiles load, but it's difficult to know when all the tiles for a source have finished loaded. Essentially, I'd like an API to know if tiles requests are in flight, or if the all the data I would expect to view for a given view is already loaded. Firing an event when the state transitions would also be helpful.

Knowing when a source is fully loaded is useful for smoothly transitioning between different overlay sources without a flash as tiles load.

As a quick sketch, I'd imaging something like:

```
map.on('source.loading', function (e) {
e.status; // one of 'fetching', 'idle', 'errored'
});
```

No files or tests are named. Start by locating the existing individual-tile load events and the source loading state or event path. Clarify the fetching, idle, and errored transitions for a source, then verify that the resulting events expose whether the expected data for the current view is loaded.

Written by the indexing model from the issue text.