A bunch of features and fixes (contains Wizzel1's scroll speed commits) [details in comment]

[alesimula]

• Include Wizzel1's scroll speed commits (but changed scrollSpeed from int to double)
• When changing scroll direction, restart animation from current position instead of future position.
• Change animation curve to Curves.easeOutQuart (looks more natural), speed to 2.0 and duration to 380
• Fix trackpad gestures not working when switching between mouse wheel and trackpad
• Fix trackpad gestures not working out of the box (it used to require a click with trackpad to work)
• Detect locked scrollbar, default behaviour for scrollview not scrollable (e.g. when the mouse is hovering the scrollbar)