add "gui log" and friends

This adds the "gui log" command, which pops up a window you can use
for logging.

There are also new "gui" subcommands for sending output to a log
window: gui print, gui output, gui printf, and gui dprintf.  These
all work pretty much like their non-gui counterparts, except that the
output goes to the log window.

If you have more than one log window you can specify which one to use
with an "@", like  gui print @1 value

gui/dprintf.py

@@ -0,0 +1,44 @@
+# Copyright (C) 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
+# 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/>.
+
+# dprintf-like machinery.
+
+import gdb
+import gui.logwindow
+
+class DPrintfBreakpoint(gdb.Breakpoint):
+    def __init__(self, spec, window, arg):
+        super(DPrintfBreakpoint, self).__init__(spec, gdb.BP_BREAKPOINT)
+        self.window = window
+        self.command = 'printf ' + arg
+
+    def stop(self):
+        window = self.window
+        if window is not None:
+            if not window.valid():
+                gdb.post_event(self.delete)
+                return False
+        else:
+            window = gui.logwindow.default_log_window
+        if window is None:
+            return False
+
+        try:
+            text = gdb.execute(self.command, False, True)
+        except something:
+            text = something
+        window.append(text)
+
+        return False