shell: Move dbus.js into private JS scope
shell: Move cockpit-plot.js into private JS scope
tools: No longer allow arbitrary 'cockpit_' globals
shell: Initialize after all javascript has been loaded

Due to the scoping we now need to have a temporary hack to
initialize once everything has loaded.

We need to come up with a better way to call an event on
a module when it has loaded.

Closes #1025