Show IME's composition text on screen.

The composition text should only show on screen temporarily and gets
disappered when input is cancelled or done. We simply mark the
composition as selected, so it can not only indicate this is composition
text instead of normal text, but also updates the composition without
adding new interfaces.

src/editor.coffee

@@ -699,7 +699,18 @@ class Editor extends View
 
   handleImeEvents: ->
     @on 'cursor:moved', =>
-      @hiddenInput.offset(@getCursorView().offset())
+      cursorView = @getCursorView()
+      @hiddenInput.offset(cursorView.offset()) if cursorView.is(':visible')
+
+    startScreenPosition = null
+    @hiddenInput.on 'compositionstart', =>
+      startScreenPosition = @getCursorScreenPosition()
+    @hiddenInput.on 'compositionupdate', (e) =>
+      @insertText(e.originalEvent.data)
+      @selectToScreenPosition(startScreenPosition)
+    @hiddenInput.on 'compositionend', =>
+      @delete()
+      startScreenPosition = null
 
   selectOnMousemoveUntilMouseup: ->
     lastMoveEvent = null