New python event "clear_objfiles".

If one is watching new_objfile events in python, it helps to know
when the list of objfiles is cleared.  This patch adds a new
clear_objfiles event to support this.

This patch is all just cut-n-paste-n-tweak derived from
the new_objfiles event.

gdb/ChangeLog:

	* NEWS: Mention new event gdb.clear_objfiles.
	* python/py-event.h (emit_clear_objfiles_event): Clear
	* python/py-events.h (events_object): New member clear_objfiles.
	* python/py-evts.c (gdbpy_initialize_py_events): Add clear_objfiles
	event.
	* python/py-inferior.c (python_new_objfile): If objfile is NULL,
	emit clear_objfiles event.
	* python/py-newobjfileevent.c (create_clear_objfiles_event_object): New
	function.
	(emit_clear_objfiles_event): New function.
	(clear_objfiles): New event.
	* python/python-internal.h (gdbpy_initialize_clear_objfiles_event):
	Declare.
	* python/python.c (_initialize_python): Call
	gdbpy_initialize_clear_objfiles_event.

gdb/doc/ChangeLog:

	* python.texi (Events In Python): Document clear_objfiles event.

gdb/testsuite/ChangeLog:

	* gdb.python/py-events.exp: Update expected output for clear_objfiles
	event.
	* gdb.python/py-events.py: Add clear_objfiles event.

gdb/python/py-events.h

@@ -46,6 +46,7 @@ typedef struct
   eventregistry_object *cont;
   eventregistry_object *exited;
   eventregistry_object *new_objfile;
+  eventregistry_object *clear_objfiles;
 
   PyObject *module;