BoltExported: Fixes and improvements: auto-string-conversion, auto-object-path creation

• Support auto object → string conversion for dbus properties: When a property is exported via BoltExported we now support automatic conversion from the object to a string (on the bus). This is done by using the object-id property of the object to be converted.

• Auto object-path generation: If the object_path is NULL in bolt_exported_export, a combination of the base object path, which can be set on BoltExportedClass and object-id of the object will be used to construct a object-path.

• We also fix holding a extra reference in BoltExported while the object is exported. Instead we unregister the object in the finalizer, to prevent any further calls being made to a dead object.