add "info windows" and "delete window"

This adds the "info windows" and "delete window" commands.

"info windows" unfortunately overlaps with the TUI's "info win".
So maybe we should rename this one.

This also adds a new "toplevel" component that knows how to track
windows.

gui/source.py

@@ -17,8 +17,10 @@
 
 import gdb
 from gui.invoker import Invoker
+from gui.toplevel import Toplevel
 import gui.startup
 import os.path
+import gui.toplevel
 
 from gi.repository import Gtk, GtkSource, GObject
 
@@ -140,8 +142,10 @@ class LRUHandler:
 
 lru_handler = LRUHandler()
 
-class SourceWindow:
+class SourceWindow(Toplevel):
     def __init__(self):
+        super(SourceWindow, self).__init__()
+
         self.frame = None
 
         self.do_step = Invoker("step")