Async load popular modes into CodeMirror

Dynamically load modes after the type of a file has been discovered
from the server. This allows browers to download only the segments
of CodeMirror they need to render the current file(s) being viewed.

Change-Id: I34c037199cd01bf65b051400999d141eb0524cd8

gerrit-gwtui/src/main/java/net/codemirror/CodeMirror.gwt.xml

@@ -17,4 +17,5 @@
   <inherits name='com.google.gwt.logging.Logging'/>
   <inherits name='com.google.gwt.resources.Resources'/>
   <source path='lib'/>
+  <source path='mode'/>
 </module>