Simplify marker events

Instead of marker-added and marker-removed events which are emitted
when markers are created/invalidated/revalidated/destroyed, we now
just have marker-created events that are triggered *only* when markers
are created for the first time. The marker itself emits a 'destroyed'
event when it is destroyed. The marker already notifies observers when
its validation status changes, so that's covered.

src/app/buffer-marker.coffee

@@ -165,6 +165,7 @@ class BufferMarker
   # Destroys the marker
   destroy: ->
     @buffer.destroyMarker(@id)
+    @trigger 'destroyed'
 
   # Returns a {Boolean} indicating whether the marker is valid. Markers can be
   # invalidated when a region surrounding them in the buffer is changed.