Method scroll

since we only have a single scroll function to work with, we do it as follows: 1. if no thumbs are active, determine which is closest to the mouse i. store the initial mouse position ii. scroll the thumb 2. if a thumb is active: i. check if we are close to the initial mouse position iia. if so, scroll the thumb iib. if not, we assume it is a new separate scroll action and go back to 1.