Add .isDestroyed method to markers

src/app/buffer-marker.coffee

@@ -168,6 +168,9 @@ class BufferMarker
   isValid: ->
     @buffer.getMarker(@id)?
 
+  isDestroyed: ->
+    not (@buffer.validMarkers[@id]? or @buffer.invalidMarkers[@id]?)
+
   ###
   # Internal #
   ###

src/app/display-buffer-marker.coffee

@@ -134,6 +134,9 @@ class DisplayBufferMarker
   isValid: ->
     @bufferMarker.isValid()
 
+  isDestroyed: ->
+    @bufferMarker.isDestroyed()
+
   destroy: ->
     delete @displayBuffer.markers[@id]
     @bufferMarker.destroy()