Begin work on adding editable shortcuts

[LorenDB]

This will eventually allow users to assign arbitrary shortcuts to actions to give them the ability to have shortcuts for everything(tm). Currently it is lacking some things like an editor though.

[LorenDB]

I'm not 100% sure that I've implemented this in the best possible way; I have to wonder if it wouldn't be better to reimplement this to have shortcuts registered during nheko startup instead of adding them on the fly as they are instantiated from QML. I'd be happy to hear feedback on that.

[duarm]

Would we be able to assign single-key hotkeys like 'i' to focus on the input field? This could be achieved by resolving the shortcuts inside Keys.onPressed on TimelineView, perhaps something like

   Keys.onPressed: event => {
        if (belongs_to_keybind(event.key)){
            begin_shortcut(event.key);
        } else if (!topBar.searchHasFocus) {
            TimelineManager.focusMessageInput();
            room.input.setText(room.input.text + event.text);
        }
   }