Windows are not released when main reference is deleted

[samschott]

Describe the bug
When a window is instantiated in an app and the reference to that window is later deleted, the actual object is never garbage collected.

To Reproduce

Here is some example code which counts the number of Window instances:

import gc
import toga
from toga.constants import COLUMN
from toga.style import Pack


class Window(toga.Window):

    instances = 0

    def __init__(self):
        super().__init__(title=f'Window: {Window.instances}', position=(200, 200))
        Window.instances += 1

    def __del__(self):
        Window.instances -= 1


class RefcountExample(toga.App):

    def startup(self):

        self.window = None

        self.button_create = toga.Button(
            'Create window',
            style=Pack(padding=10, width=120),
            on_press=self.on_create,
        )

        self.button_delete = toga.Button(
            'Close and delete window',
            style=Pack(padding=10, width=120),
            on_press=self.on_delete,
        )

        self.label = toga.Label(
            f'Instances: {Window.instances}',
            style=Pack(padding=10, width=120),
        )

        self.button_create.enabled = True
        self.button_delete.enabled = False

        self.box = toga.Box(
            children=[
                self.button_create,
                self.button_delete,
                self.label,
            ],
            style=Pack(direction=COLUMN),
        )

        self.main_window = toga.MainWindow()
        self.main_window.content = self.box
        self.main_window.show()

    def on_create(self, sender):
        if not self.window:
            self.window = Window()
            self.app.windows += self.window
            self.window.content = toga.Box()
            self.window.show()
            self.label.text = f'Instances: {Window.instances}'

        self.button_create.enabled = False
        self.button_delete.enabled = True

    def on_delete(self, sender):
        if self.window:
            self.window.close()
            self.window = None
            gc.collect()  # clean up ref cycles before displaying instance count
            self.label.text = f'Instances: {Window.instances}'

        self.button_create.enabled = True
        self.button_delete.enabled = False


def main():
    return RefcountExample('Example', 'org.beeware.refcount')


if __name__ == '__main__':
    app = main()
    app.main_loop()

Expected behavior
Even if circular references to the window object exist, I would expect them to be garbage collected.

Environment:

• Operating System: macOS
• Python version: 3.9
• Software versions:
  • Briefcase:
  • Toga: 0.3.0.dev27

Additional context
The implications could be worse than they initially seem since all window content will be retained as well and can potentially use a log of memory (imagine images or a video playing in the window),.