Fix double invocation of App._stop when using App.stop and App.run

[CoreTaxxe]

Fixes https://github.com/kivy/kivy/issues/2397 and https://github.com/kivy/kivy/issues/4577

Previosly on_stop - or App._stop in general - would be triggered twice if an App instance was used in combination with App.run and App.stop. This was due to App.run invoking App._stop on its own as well.

My proposed fix keeps this behaviour as is but changes App.stop to not call App._stop directly but instead use stopTouchApp or EventLoop.close to have thenApp.run invoke App._stop

This should not cause any issues since App._stop is only ever invoked by App.run, App.async_run and previously App.stop as far as I am aware and running App.run still yield in the same results even if App.stop is never called by the user. The only change in behaviour would be using App.stop without ever calling App.run which shouldn't ever be the case anyways.

This also voids the second stopTouchApp call in App._stop

MRE

from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
Button:
    text: 'Exit'
    on_release: app.stop()
''')


class TestApp(App):
    def build(self):
        return root

    def on_stop(self):
        print('stopping')


if __name__ == '__main__':
    TestApp().run()

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• [ ] Unittests are included in PR.
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