Ensure we don't try to draw a destroyed selection on editor refresh

Destroying the cursor will cause a redraw, which causes a problem when
we execute it synchronously in specs because destroying the cursor also
destroys the marker associated with the selection. Here we remove the
selection first to avoid this problem.

src/app/selection.coffee

@@ -20,11 +20,11 @@ class Selection
       @destroy()
 
   destroy: ->
-    if @cursor
-      @cursor.off('.selection')
-      @cursor.destroy()
+    return if @destroyed
+    @destroyed = true
     @editSession.removeSelection(this)
     @trigger 'destroyed'
+    @cursor?.destroy()
 
   finalize: ->
     @initialScreenRange = null unless @initialScreenRange?.isEqual(@getScreenRange())