Update for shoes-dsl

Shoes::TextBlock has changed so that we can't set backend text in
This implementation will obviously only handle the very simplest
case.
Extract IPC, rework size messages
Extract browser, atom backends

Default to using 'browser' backend. Atom backend is a special case. Use
the lib/bootstrap files to selectively load browser/atom classes. Atom
can really reuse most of the browser classes without change, so just
assign those Shoes::Atom::Klass constants to Shoes::Browser::Klass.