Use run_on_main_thread in gdb.post_event

Use run_on_main_thread in gdb.post_event

This changes gdb.post_event to use the new run_on_main_thread
function.  This is somewhat tricky because the Python GIL must be held
while manipulating reference counts.

2019-11-26  Tom Tromey  <tom@tromey.com>

	* python/python.c (class gdbpy_gil): New.
	(struct gdbpy_event): Add constructor, destructor, operator().
	(gdbpy_post_event): Use run_on_main_thread.
	(gdbpy_initialize_events): Remove.
	(do_start_initialization): Update.

Change-Id: Ie4431e60f328dae48bd96b6c6a8e875e70bda1de