Expose a "statechange" event on GeolocateControl

Nobody has claimed this yet.

## Motivation

Discussion started here: https://github.com/mapbox/mapbox-gl-js/pull/4479#issuecomment-321203924

The `GeolocateControl` doesn't expose events necessary to act on all of its states. `trackuserlocationstart` and `trackuserlocationend` aren't enough to detect some user actions. For instance, it's impossible to know if the user toggled off tracking.

To achieve this I currently have to use a `MutationObserver` on the `.mapboxgl-ctrl-geolocate` watching for `classList` changes.

## Design

Fire a new `statechange` event on every `_watchState` change and pass along its value.

### Implementation

Replace all `this._watchState = ...` with:
```javascript
_setState(state) => {
if (this._watchState !== state) {
this._watchState = state
this.fire('statechange', state)

// ... conditions logic to fire trackuserlocationstart and trackuserlocationend

// bonus?
this._updateUI(state) // centralize the different classList updates spread in _onSucces, _onError, _onClick...
}
}
```

Start at the GeolocateControl entry point and inspect each _watchState assignment and the existing control tests. Done means a statechange event is emitted for every state transition and carries the new state, without losing the existing tracking events.

Written by the indexing model from the issue text.