Abiathar Online - Remember the scroll position

The real Abiathar remembers the viewport positioning of each level when switching between them. That's especially important when going to the tileset to fetch a tile; resetting the level view back to the upper left upon return is jarring. So today I made Abiathar Online keep track of the scrollbar positions for each level and tileset. It required a little refactoring to make sure that all "change the current level" calls go through the part that stores the current X and Y in the level, but overall it was an easy and worthwhile change, and is now live on GitHub.