Add ShumateSimpleMap

This widget is designed for simple use cases of libshumate, mainly in apps where maps aren't the main focus. It takes care of license text, source switching, base vs. overlay layers, etc. but offers somewhat less control. The individual child widgets that make up the widget are still accessible.

Also removes the custom styling on the compass widget, because it doesn't look right where I put it in the ShumateSimpleMap.

Fixes #20 (closed).