changes in osu-framework

• Add OnlineStore to AudioManager stores #901 (DrabWeb)
• Remove all unnecessary parenthesis where object initialisers are used #899 (peppy)
• Remove all framework usage of RelativeMouseDrag #898 (peppy)
• Various framework improvements #896 (Tom94)
• Disallow disposal of TextureWhitePixel #895 (Tom94)
• Make Container implement ICollection and IReadOnlyList #893 (Tom94)
• Expose TrackVirtual #891 (peppy)
• Document Damp function and fix its behavior #890 (Tom94)