Implement a system for easily registering scoped shortcuts.

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Choices:

• https://github.com/electron/electron/blob/master/docs/api/accelerator.md
• https://github.com/ccampbell/mousetrap

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(old note in the Keyboard shortcuts project converted to an issue in light of #130)

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Mousetrap has now been added by @CAYdenberg in #130.

There's still some discussion to be had around how to generalise registration of shortcuts, but for the time being keyboard shortcuts can be implemented by using mousetrap directly - see #133 for an example.