Resolve "scroll position problem in Courseware's ribbon", fixes #467

This PR fixes #467 (closed) by introducing a new way to store and read the scroll positions for each tool. Main Changes

• ref="ribbonContent" attribute was incorrect.
• v-show instead of v-if, in order to have the element in DOM instead of destroying and recreating it.
• handleScroll method is introduced in order to save the current scroll position of each tool upon scrolling, it has its own debouncing mechanism.
• updateScrollPos method is introduced to perform the scroll with animation after the nextTick!