gdb/python: deallocate tui window factories at Python shut down

The previous commit relied on spotting when a Python defined TUI
window factory was deleted.  I spotted that the window factories are
not deleted when GDB shuts down its Python environment, they are only
deleted when one window factory replaces another.  Consider this
example Python script:

  class TestWindowFactory:
      def __init__(self, msg):
          self.msg = msg
          print("Entering TestWindowFactory.__init__: %s" % self.msg)

      def __call__(self, tui_win):
          print("Entering TestWindowFactory.__call__: %s" % self.msg)
          return TestWindow(tui_win, self.msg)

      def __del__(self):
          print("Entering TestWindowFactory.__del__: %s" % self.msg)

  gdb.register_window_type("test_window", TestWindowFactory("A"))
  gdb.register_window_type("test_window", TestWindowFactory("B"))

And this GDB session:

  (gdb) source tui.py
  Entering TestWindowFactory.__init__: A
  Entering TestWindowFactory.__init__: B
  Entering TestWindowFactory.__del__: B
  (gdb) quit

Notice that when the 'B' window replaces the 'A' window we see the 'A'
object being deleted.  But, when Python is shut down (after the
'quit') the 'B' object is never deleted.

Instead, GDB retains a reference to the window factory object, which
forces the Python object to remain live even after the Python
interpreter itself has been shut down.

The references themselves are held in a dynamically allocated
std::unordered_map (in tui/tui-layout.c) which is never deallocated,
thus the underlying Python references are never decremented to zero,
and so GDB never tries to delete these Python objects.

This commit is the first half of the work to clean up this edge case.

All gdbpy_tui_window_maker objects (the objects that implement the
TUI window factory callback for Python defined TUI windows), are now
linked together into a global list using the intrusive list mechanism.

When GDB shuts down the Python interpreter we can now walk this global
list and release the reference that is held to the underlying Python
object.  By releasing this reference the Python object will now be
deleted.

I've added a new assert in gdbpy_tui_window_maker::operator(), this
will catch the case where we somehow end up in here after having
reset the reference to the underlying Python object.  I don't think
this should ever happen though as we only clear the references when
shutting down the Python interpreter, and the ::operator() function is
only called when trying to apply a new TUI layout - something that
shouldn't happen while GDB itself is shutting down.

This commit does not update the std::unordered_map in tui-layout.c,
that will be done in the next commit.

Reviewed-By: Tom Tromey <tom@tromey.com>

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