add frame support

if you have a gdb that can wrap commands (not yet upstream)
then up/down/frame commands will update the source window
when a selected-frame event is available, we can change
the code to use that instead

gui/events.py

@@ -0,0 +1,32 @@
+# Copyright (C) 2013 Tom Tromey <tom@trolley.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
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import gdb
+
+class _Event(object):
+    def __init__(self):
+        self.funcs = []
+
+    def connect(self, callback):
+        self.funcs.append(callback)
+
+    def disconnect(self, callback):
+        self.funcs.remove(callback)
+
+    def post(self):
+        for fun in self.funcs:
+            fun()
+
+frame_changed = _Event()