Gtk.ToolButton expects a Gtk.Image for the icon, not a GdkPixbuf.Pixbuf

[rmartin16]

Describe the bug

The icon for a command on a toolbar throws this error:

[gtktest] Starting in dev mode...
===========================================================================
Traceback (most recent call last):
  File "/home/russell/github/beeware/briefcase/venv-3.10-briefcase/lib/python3.10/site-packages/toga_gtk/app.py", line 98, in gtk_startup
    self.interface._startup()
  File "/home/russell/github/beeware/briefcase/venv-3.10-briefcase/lib/python3.10/site-packages/toga/app.py", line 556, in _startup
    self.startup()
  File "/home/russell/tmp/beeware/gtktest/src/gtktest/app.py", line 35, in startup
    self.main_window.toolbar.add(cmd1)
  File "/home/russell/github/beeware/briefcase/venv-3.10-briefcase/lib/python3.10/site-packages/toga/command.py", line 393, in add
    self.on_change()
  File "/home/russell/github/beeware/briefcase/venv-3.10-briefcase/lib/python3.10/site-packages/toga_gtk/window.py", line 82, in create_toolbar
    item_impl.set_icon_widget(cmd.icon._impl.native_32)
TypeError: argument icon_widget: Expected Gtk.Widget, but got gi.overrides.GdkPixbuf.Pixbuf

Steps to reproduce

Sample app named gtktest

import toga
from toga.style import Pack
from toga.style.pack import COLUMN, ROW


class gtktest(toga.App):

    def startup(self):
        stuff_group = toga.Group('Stuff', order=40)
        cmd1 = toga.Command(
            lambda: print(self),
            text='Example command',
            tooltip='Tells you when it has been activated',
            shortcut='k',
            icon='resources/gtktest.png',
            group=stuff_group,
            section=0
        )

        self.main_window = toga.MainWindow(title=self.formal_name)

        self.commands.add(cmd1)
        self.main_window.toolbar.add(cmd1)

        main_box = toga.Box()
        self.main_window.content = main_box
        self.main_window.show()


def main():
    return gtktest()

Expected behavior

Icon renders without an error.

Screenshots

No response

Environment

• Operating System: pop os 22.04
• Python version: 3.10.12
• Software versions:
  • Briefcase: 0.3.16.dev278+gf732c612.d20231010
  • Toga: 0.3.2.dev1256+gedb92eb5

Logs

briefcase.2023_10_10-16_19_32.dev.log

Additional context

Initial report: https://discord.com/channels/836455665257021440/836455665257021443/1161383265698070700

[freakboy3742]

I can confirm this in the morning, but I think this has been addressed by #2075.