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

css/app.css

@@ -1415,6 +1415,14 @@ div.combobox {
     height:18px;
 }
 
+.background-control .layer-toggle-gpx .layer-extent {
+    display:none;
+}
+
+.background-control .layer-toggle-gpx.selected .layer-extent {
+    display:inline-block;
+}
+
 /* Geocoder */
 
 .geocode-control, .geocode-control form {
@@ -1471,9 +1479,12 @@ div.combobox {
     background:#000;
 }
 
-#surface, #tile-g {
+#surface, #layer-g, .layer-layer {
     position:absolute;
     top:0;
+    left: 0;
+    right: 0;
+    bottom: 0;
     transform-origin:0 0;
     -ms-transform-origin:0 0;
     -webkit-transform-origin:0 0;
@@ -1489,10 +1500,6 @@ div.combobox {
   position: static;
 }
 
-#tile-g {
-  opacity: 0.5;
-}
-
 /* About Section
 ------------------------------------------------------- */