Allow successive accented characters to be inserted in React editor

Refs #2732

Because we're only checking the length of the input element's selection
in the React editor on textinput events and not also its content, we
were mistaking some IME compositions as accented character menu
insertions. Clearing the content of the input on 'compositionend'
prevents this issue.

src/editor-component.coffee

@@ -356,12 +356,13 @@ EditorComponent = React.createClass
     #   5. textInput fired; event.data == the completion string
 
     selectedText = null
-    node.addEventListener 'compositionstart', =>
+    node.addEventListener 'compositionstart', ->
       selectedText = editor.getSelectedText()
-    node.addEventListener 'compositionupdate', (event) =>
+    node.addEventListener 'compositionupdate', (event) ->
       editor.insertText(event.data, select: true, undo: 'skip')
-    node.addEventListener 'compositionend', =>
+    node.addEventListener 'compositionend', (event) ->
       editor.insertText(selectedText, select: true, undo: 'skip')
+      event.target.value = ''
 
   listenForCommands: ->
     {parentView, editor, mini} = @props