Support adding map-sources after initialization

[jimk]

Support adding map-sources after initialization by separating out the inner loop of Application.configureMapSources into its own function and creating a public API for that function. This is to allow extensions to be able to define new layers and add them to the map.

I don't believe these changes will effect existing uses of the application.

[theduckylittle]

r931

[jimk]

Works, thanks!