gdb/python: Use copy-initialization more when possible

gdb/python: Use copy-initialization more when possible

gdb/ChangeLog:
2018-08-24  Pedro Alves  <palves@redhat.com>

	* python/py-bpevent.c (create_breakpoint_event_object): Use
	copy-initialization.
	* python/py-continueevent.c (emit_continue_event): Use
	copy-initialization.
	* python/py-exitedevent.c (create_exited_event_object): Return a
	gdbpy_ref<>.
	(emit_exited_event): Use copy-initialization.
	* python/py-inferior.c (python_new_inferior)
	(python_inferior_deleted, add_thread_object): Use
	copy-initialization.
	* python/py-infevents.c (create_inferior_call_event_object)
	(create_register_changed_event_object)
	(create_memory_changed_event_object): Return a gdbpy_ref<>.
	(emit_inferior_call_event, emit_memory_changed_event)
	(emit_register_changed_event): Use copy-initialization.
	* python/py-newobjfileevent.c (create_new_objfile_event_object):
	Return a gdbpy_ref<>.
	(emit_new_objfile_event): Use copy-initialization.
	(create_clear_objfiles_event_object): Return a gdbpy_ref<>.
	(emit_clear_objfiles_event): Use copy-initialization.
	* python/py-signalevent.c (create_signal_event_object): Use
	copy-initialization.
	* python/py-threadevent.c (create_thread_event_object): Use
	copy-initialization.