Add custom input mapping

[Pandawan]

This is more of a long-term thing, but it would be cool to have a config that maps specific inputs/buttons to actions.

For example, the play video action could be mapped to "space" on keyboard, but if someone is using a controller, they could specify "A button" instead; similarly, on a remote, if the remote registers its "play button" as "Enter" you could map "Enter" to play video as well.

This would not only allow for custom controls, and would make it relatively easier to develop support for all three input types (keyboard, remote, controller).

To make it simpler, you could also have basic presets in a dropdown with options keyboard, remote, controller, and custom where custom allows you full control over every action.

[jonbarrow]

The beginning of this has been added with keybinds.json. I will keep this issue open though as the full feature request has not been fulfilled.