Allow connecting one client to multiple Flexx servers

[almarklein]

In other words, multiple Flexx "instances" on the same page. A direct (real) use-case is that in JLab multiple notebooks are open in which Flexx is used (and perhaps some JLab plugins run on Flexx as well).

Aspects:

• The Flexx object that is now on window.flexx should be in a module, or somehow support multiple sockets.
• Model id's need to not clash.
• BTW: on the server side, the counter for object id's should be on the session, not on global.
• Also see the window.flexx.get() method, which is hardly used.

[almarklein]

#408 put one step in this direction, but more work would be needed to make it really work.