Add sliding anchor along adjacent segments in path tool

[4adex]

Part of #1870

‼️CtrlShift (updated since video was recorded which indicated this would use Alt) slides the anchor along its adjacent segment(s) while adjusting its handle(s) to best approximate the initial curvature. Since only the two handles may vary in length >= 0 and it must pass through the sliding point's starting position, there's only one dimension of freedom and a simple Newton's method optimization for error based on Hausdorff or Frechet distance should be fine, as far as the algorithm goes. (If pressed before the drag begins, it shouldn't break Shift-clicking points to add to the selection.)