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.
bridge: Add support for flags in dbus messages

The only thing we do now is report the endianness of the message
as a flag. But in the future this is where things like the new
PERMIT_INTERACTIVE_AUTHORIZATION flag will go.

Fixes #1358
bridge: Implement support for monitoring properties

 * Watch support for monitoring DBus properties.
 * Introspection meta data suppport.
 * Proxies, and caching of data in javascript.
 * Ordering guarantees for method calls vs. interfaces appearing,
   including signals, error returns, etc..

Closes #1356