cockpit: Add cockpit_json_parse_object() method

And make cockpit_json_parse_bytes() return a JsonObject. It's by far
the most common situation that we're parsing and writing JsonObject
rather than individual JsonNode. So make those cases realy easy.

Related to #413
Closes #508

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
- Removed a merge conflict header
- Added an = NULL in cockptidbusjson.c