Makes UiBinder able to deal with mixed content in ui:msg.

E.g.:

<p><ui:msg>
  You can mix both widgets and named dom fields within a
  <gwt:HTML ui:field='mixedMessageWidget'>single
  translatable message</gwt:HTML>, another source
  <span ui:field='mixedMessageSpan'>exciting and
  subtle</span> bugs.
</ui:msg></p>

Reviewed by bobv

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6750 8db76d5a-ed1c-0410-87a9-c151d255dfc7