base: Implement a javascript client for new dbus-json3

This implements a simple javascript client for dbus-json3. It has
no proxy support at this time, but that will be added later.

Fixes #1352
Fixes #1355
doc: Add documentation for cockpit.dbus and related

Closes #1356
bridge: Add explicit 'type' support for dbus-json3

You can specify the 'type' of a call, and then no Introspect() will
be needed when calling the method. In addition, the 'type' will be
returned in the reply.