Fix gst_element_factory_make function. Add callback-function for 'notify' signal of the created pipeline-object's proxy
Add new stub type - GstBus
Add GstBus stub type, functions for invoking gstd methods of FactoryInterface and PipelineInterface. Also, implement audioplayer using gstd. Signals don't work yet, probably this is gstd bag.
Change gst_element_set_remote_property to use 'pspec' parameter, gst_element_factory_make to use 'factoryname' param. Also, change freeing in factory_interface.