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.

src/app/display-buffer-marker.coffee

@@ -15,6 +15,7 @@ class DisplayBufferMarker
 
   constructor: ({@bufferMarker, @displayBuffer}) ->
     @id = @bufferMarker.id
+    @bufferMarker.on 'destroyed', => @destroyed()
 
   ###
   # Public #
@@ -155,6 +156,9 @@ class DisplayBufferMarker
   # Internal #
   ###
 
+  destroyed: ->
+    delete @displayBuffer.markers[@id]
+
   observeBufferMarkerIfNeeded: ->
     return if @subscriptionCount()
     @getHeadScreenPosition() # memoize current value

src/app/display-buffer.coffee

@@ -33,12 +33,7 @@ class DisplayBuffer
     @tokenizedBuffer.on 'grammar-changed', (grammar) => @trigger 'grammar-changed', grammar
     @tokenizedBuffer.on 'changed', @handleTokenizedBufferChange
     @buffer.on 'markers-updated', @handleMarkersUpdated
-    @buffer.on 'marker-added', (marker) =>
-      @trigger 'marker-added', @getMarker(marker.id)
-    @buffer.on 'marker-removed', (marker) =>
-      marker = @getMarker(marker.id)
-      delete @markers[marker.id]
-      @trigger 'marker-removed', marker
+    @buffer.on 'marker-created', (marker) => @trigger 'marker-created', @getMarker(marker.id)
 
   buildLineMap: ->
     @lineMap = new LineMap

src/app/text-buffer.coffee

@@ -476,7 +476,7 @@ class Buffer
       attributes
     }, options))
     @validMarkers[marker.id] = marker
-    @trigger 'marker-added', marker
+    @trigger 'marker-created', marker
     marker
 
   # Public: Constructs a new marker at a given position.
@@ -650,7 +650,6 @@ class Buffer
     if marker = @validMarkers[id] ? @invalidMarkers[id]
       delete @validMarkers[id]
       delete @invalidMarkers[id]
-      @trigger 'marker-removed', marker
 
   scheduleModifiedEvents: ->
     clearTimeout(@stoppedChangingTimeout) if @stoppedChangingTimeout