Pyscript should have a (virtual) namespace for JS stuff about flexx

[almarklein]

I eventually went with window, because it (historically) already is the global variable in the browser. It is thus already familiar, we don't have to do anything in the transpilation, it seems special enough to avoid name clashes, yet simple enough to not get too annoyed from typing it a lot.

Flexx creates a window alias for root on Nodejs, so that the same name can be used on node.