Multiple shortcut keys for a single command

I just tried Podium in Python3.7 on Ubuntu 18.04, it worked great.

There was no support for my editor's bindings though, which I think is because [`toga.command.Command`](https://toga.readthedocs.io/en/latest/reference/api/resources/command.html) only has one `shortcut` attribute. I propose changing it to a `shortcuts` collection. It could retain the `shortcut` api for backward compatibility.

Start with the toga.command.Command API described in the linked reference documentation and trace where its single shortcut attribute is consumed. Determine the affected command and platform code and any existing tests before defining the collection behavior. The work is done when multiple bindings are supported while the existing shortcut API remains compatible.

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