Get rid of NSRulerView

Our NSRulerView subclass doesn’t use much behaviour from NSRulerView since we’re drawing markers and labels ourselves. Furthermore, NSScrollView with visible rulers has magnification problems, which has led us to implement magnification manually, which in turn is related to #80.

Another approach would be a custom view containing a scroll view and two (maybe four) ruler views that do not inherit from NSRulerView. This is only slightly more complex than our current design, and it will potentially fix the magnification issues.

This would also help #79 and #81.