gdb/python: allow Python TUI windows to be replaced

gdb/python: allow Python TUI windows to be replaced

The documentation for gdb.register_window_type says:

  "...  It's an error to try to replace one of the built-in windows,
  but other window types can be replaced. ..."

I take this to mean that if I imported a Python script like this:

  gdb.register_window_type('my_window', FactoryFunction)

Then GDB would have a new TUI window 'my_window', which could be
created by calling FactoryFunction().  If I then, in the same GDB
session imported a script which included:

  gdb.register_window_type('my_window', UpdatedFactoryFunction)

Then GDB would replace the old 'my_window' factory with my new one,
GDB would now call UpdatedFactoryFunction().

This is pretty useful in practice, as it allows users to iterate on
their window implementation within a single GDB session.

However, right now, this is not how GDB operates.  The second call to
register_window_type is basically ignored and the old window factory
is retained.

This is because in tui_register_window (tui/tui-layout.c) we use
std::unordered_map::emplace to insert the new factory function, and
emplace doesn't replace an existing element in an unordered_map.

In this commit, before the emplace call, I now search for an already
existing element, and delete any matching element from the map, the
emplace call will then add the new factory function.

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