add thread enforcement decorators

This adds a couple of decorators that can be used to
mark functions as needing to be run in the gdb thread
or in the gtk thread.

This found at least one bug.

gui/commands.py

@@ -1,4 +1,4 @@
-# Copyright (C) 2012 Tom Tromey <tom@tromey.com>
+# Copyright (C) 2012, 2013 Tom Tromey <tom@tromey.com>
 
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -21,6 +21,7 @@ import gui.toplevel
 import gui.dprintf
 import gui.events
 import re
+from gui.startup import in_gtk_thread
 
 class GuiCommand(gdb.Command):
     def __init__(self):