Make idempotent add/update layer style functions

[stevage]

Not sure what these would be called exactly, but it would be really convenient to have one function which defines a layer or source, and can be repeatedly called when values change.

Syntax maybe something like:

map.U.lineLayer('myline', 'mysource', {
...
});

So the first time it creates the layer. Subsequent calls just apply all the properties individually.

[stevage]

Some tricky considerations:

• properties like filter can be set, but not in the same way
• properties like source-layer can't be changed - either recreate the whole layer, or just warn.

[stevage]

Actually, I'm going to be even bolder:

addLineLayer() etc will now be idempotent, so this simply adds a layer, then changes the definition of that layer:

map.U.addCircleLayer('mycircles', 'mysource', { circleStrokeColor: 'red' });
// if the layer already exists, calling add*Layer simply updates any of the properties
map.U.addCircleLayer('mycircles', 'mysource', { circleStrokeColor: 'red', circleRadius: 4, filter: ['==', 'type', 'active'});

And the same goes for addGeoJSON etc:

map.U.addGeoJSON('mysource');
map.U.addGeoJSON('mysource', geojson);

I think this is going to make working with layers a lot less painful, and remove a lot of race conditions, annoying warnings about "source already exists" etc.

Instead of having to write one set of code that creates layers, and another set that updates specific properties, you can just write one function that defines all the layers (as a function of application state), and call that whenever application state changes.

(It turned out to be pretty easy to write using map.setStyle().)