Support to get stylable properties of requested class only or of all parent classes as well

xfdashboard_actor_get_stylable_properties is splitted into two functions: xfdashboard_actor_get_stylable_properties and xfdashboard_actor_get_stylable_properties_full. The first one will fill the hashtable only with stylable properties of requested and the latter one will also get the stylable properties of all parent classes recursively.