Add support for zIndex when customizing layers.

[fschmenger]

When testing the background layer picker, I realized that we're currently relying on order of layer declaration to define which layers should appear in foreground. This is somewhat inconvenient when 'baseLayers' are declared at the wrong end of the 'mapLayers' list.

• I suggest to generally support a 'zIndex' configuration property for 'mapLayers'.
• There should be some implicit 'zIndex' assigned when the property is missing, such that 'baseLayers' will get a lower zIndex than foreground layers. What numbers should we use here?