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/testsuite/gdb.python/py-events.py

@@ -57,6 +57,11 @@ def new_objfile_handler (event):
     print ("event type: new_objfile")
     print ("new objfile name: %s" % (event.new_objfile.filename))
 
+def clear_objfiles_handler (event):
+    assert (isinstance (event, gdb.ClearObjFilesEvent))
+    print ("event type: clear_objfiles")
+    print ("progspace: %s" % (event.progspace.filename))
+
 class test_events (gdb.Command):
     """Test events."""
 
@@ -80,6 +85,7 @@ class test_newobj_events (gdb.Command):
 
     def invoke (self, arg, from_tty):
         gdb.events.new_objfile.connect (new_objfile_handler)
+        gdb.events.clear_objfiles.connect (clear_objfiles_handler)
         print ("Object file events registered.")
 
 test_newobj_events ()