Polygon Editing Improvements

Since the last snapshot I've made several improvements to the polygon editing functionality. In particular, it is now possible to interact with the segments in addition to the points. This enables several convenient actions:

• Click a segment to select the handles on both ends. As usual, holding Shift will add to the existing selection.
• Right-click a segment to select the handles on both ends and open the context menu.
• Drag a segment around.
• Double-click a segment to split it at the clicked location. Hold down the mouse button to immediately drag the new point to where you need it.