More path tools

[pasbi]

• [x] Knife, which will add a new point between points many more tools are possible.
• [x] draw path to create a new path. Click-release to draw nibbed point or click-move-release to draw point with tangents
• [ ] merge points drag-and-drop points on other points to merge them (come up with sth. smart for non-neighbors)
• [ ] bevel split a point into, control the distance between the two on the old path using the mouse. Optionally smoothen the segment between the new points.
• [ ] points-dialog: right click on point shows context menu with the option to open a point dialog. One can edit the position of the point (cartesian) as well as its tangents (cartesian and polar)