* Fixed opacity control
* Reorganize layer switcher
* Add GPX item to menu
* Allow user to cinch to GPX extent
Needs:
* Moderate refactoring
* Possibly a specific 'layerchange event'
* Dropping GPX on the map should be a behavior, probably, instead of in
the layer
* Layers, if we want them, should be named rather than just have list
indices
* A better icon for zoom to extent and for that icon to be properly
placed
