Add setOnMapLoadedCallback like in GoogleMap

[afarber]

Fixes #59

Adds OpenMapView.setOnMapLoadedCallback(OnMapLoadedCallback?), matching Google's GoogleMap.setOnMapLoadedCallback.

• At the end of MapController.draw() the callback is called once if every visible base tile was drawn from memory and the camera isn't moving. An expired tile shown as offline fallback counts as drawn, and with MapType.NONE there is nothing to wait for
• It runs on the main thread after the draw pass, so it can safely call snapshot() or change the map
• Setting it on a map that is already loaded requests a redraw, which then reports it
• Tile overlays are not waited for, because sparse overlays (e.g. OpenSeaMap) have no tiles for many areas and the callback would never fire
• Like Google's, it doesn't fire while tiles can't be loaded or while the camera keeps moving
• null removes a pending callback

Example:

mapView.setOnMapLoadedCallback {
    mapView.snapshot { bitmap -> bitmap?.let { saveToFile(it) } }
}

Tests: new MapLoadedCallbackTest (7 tests). 489 tests pass, coverage 83%. PUBLIC_API.md marks it as implemented.