Customizing shortcuts

It would be useful for the user to customise shortcuts, which could probably be saved in the local storage of the browser.

This could probably be added to the `file -> preferences` menu. All of the shortcuts are currently hardcoded in `default_mapping.rs` - however it may be necessary to filter these keybinds before displaying them (e.g. it doesn't make sense for the user to need to customise each key press for nudge, and pointer move events should not change).