Allow plugins to add keyboard shortcuts

[evankford]

Story:
Plugins should be able to add keyboard shortcuts. They should also receive a signal on plugin load whether the shortcut was registered to help with handling errors (e.g. a plugin that just adds a shortcut to some function could fail, but others could work fine without the shortcut, or even potentially try a different chord).

Implementation details:
I believe it would be relatively trivial to expose the ability to register shortcuts into the plugin loop and simply report a validation warning back to the plugin if Paseo cannot register. It obviously would require adding some new functionality to the plugin register functions and exposing that functionality and hooking into the core Paseo app registration. I do not think there would be any implementation effect on mobile apps

Secondary Reasoning:
I think it would be really valuable for the development of Paseo to be able to add plugins that can create almost entire features for Paseo in order to get them tested before merging into the main feature. For instance, I really want to be able to mark PR or file changes as viewed using keyboard shortcuts, etc. Something that I'd like to be in the final version of the app, but can be tested as a plug-in first.

I'm happy to open a PR if this sounds like a good idea.