Make private bindable `readonly`
Make private bindable `readonly`

ppy/osu-framework,ZLima12/osu-framework,peppy/osu-framework,ppy/osu-framework,peppy/osu-framework,smoogipooo/osu-framework,ppy/osu-framework,ZLima12/osu-framework,smoogipooo/osu-framework,peppy/osu-framework