[gwt-contrib] Adds a __gwt_disconnected function to hosted.html, which glasses the screen and puts up a disconnect

[scottb]

Reviewers: jat,

Description:
This function is to be called from the GWT dev plugin.

Also removes legacy support from hosted.html.


Please review this at http://gwt-code-reviews.appspot.com/109802

Affected files:
   dev/core/src/com/google/gwt/core/ext/linker/impl/hosted.html


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