PR python/13598 - add before_prompt event

This adds an event that is emitted just before GDB presents a prompt
to the user.  This provides Python code a way to react to whatever
changes might have been made by the previous command.  For example, in
my GUI I use this to track changes to the selected frame and reflect
them in the UI.

Built and regtested on x86-64 Fedora 23.

gdb/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* python/python.c (gdbpy_before_prompt_hook): Emit before_prompt
	event.
	* python/py-evts.c (gdbpy_initialize_py_events): Add
	before_prompt registry.
	* python/py-events.h (events_object) <before_prompt>: New field.

gdb/doc/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* python.texi (Events In Python): Document events.before_prompt.

gdb/testsuite/ChangeLog
2017-02-14  Tom Tromey  <tom@tromey.com>

	PR python/13598:
	* gdb.python/py-events.exp: Add before_prompt event tests.