Local GPX file support, first push towards multilayer support.
Merge branch 'master' into layers
Refactoring and fixing layers

* 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
Fix tests
Merge branch 'master' into layers
Detect file and drag and drop support
Merge pull request #1093 from systemed/layers

Layers