IE, Chrome, Firefox plugins: gracefully disconnect when server connection drops.

1) Plugins fails gracefully in the face of a disconnect, returning undefined instead of making lots of noise.

2) Plugins invokes hosted.html's __gwt_disconnected() method the first time the session is detected as dropped.  This glasspanels the app.

Review by: jat

git-svn-id: 8db76d5a-ed1c-0410-87a9-c151d255dfc7
12 files changed