feat: adding unbind methods to App

[etyloppihacilem]

Adding a method to App that allows to unbind a key previously bound.

class MyApp(App):
    def on_mount(self):
        self.bind("a", action="action1", description="description1")

app = MyApp()

app.unbind("a")  # removes the 'a' binding

Please review the following checklist.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

I don't know if an update to documentation is needed as I correctly (ig ?) wrote the docstrings of the methods written in app.py and binding.py...

This may be linked to #2242 but disabling with unbind may be more permanent.