Allow moving neighbouring thumbs to accommodate new thumb values (for issue #55)

[Dretch]

This only addresses the first use case in issue #55.

I don't really understand the second use case: why would you want to allow breaking the min/max/step invariants in this case? I don't see any other situations where it is allowed to break these invariants.

I choose to augment the API to implement this feature by overloading Thumb::setValue with a version taking a "push" flag. This might not be the best approach, especially if use case 2 is needed. It might be better to have differently named methods, or to replace the flag with an enum.

Please let me know what you think. :)

[leokraemer]

I like the feature. Please consider accepting the pull-request.