Refactor absolute scrolling at song select

[peppy]

This is a bit of a catch-all fix up while implementing this in the new carousel.

• Adds support for absolute scrolling to carousel v2
• User can now choose a custom binding instead of right mouse if you wish
• Removes most of the handling code from OsuScrollContainer
• Removes the setting. This can now be disabled by unbinding the key. Note that this changes the default to have it as right mouse button. This is an intentional change – I believe this functionality should be on by default as it's beneficial to most users.