Added auto-scroll, scroll prevention (to help Vue2-Dragula on mobile), size recalculation on component updates

[tmcdos]

1. scrollbar size is recalculated when the component is updated (usually content is added or removed)
2. width of horizontal scrollbar is reduced from 100% when the vertical scrollbar is visible to avoid their mutual overlapping
3. height of vertical scrollbar is reduced from 100% when the horizontal scrollbar is visible to avoid their overlapping
4. scrollbar position is properly updated when it was at 100% and inner content was removed (otherwise you can no longer scroll - the slider gets stuck at the end)
5. got rid of document listeners - they are needless as we only want to scroll our container, not the whole page
6. added ability for auto-scrolling (e.g. when used in combination with Vue2-Dragula)
7. added ability to prevent scrolling on mobile (e.g. when dragging items through Vue2-Dragula) - otherwise the scrolling takes precedence over dragging (on mobile only)
8. added computed properties to show whether the horizontal or vertical scrollbars are currently visible
9. the component emits events during scrolling - "scrollx" for horizontal, "scrolly" for vertical and "scroll" for any of them