Add support for relative and absolute coordinates to be entered#13

related to: https://github.com/bertyhell/openwebcad/issues/12 When a tool expects a point to be clicked, you can also enter coordinates for that point using the keyboard. eg: * select the line tool * enter `100,100` * the line tool interprets that as a click on coordinate `100,100` and takes that point as the start point of the line * enter `200,100` * the line tool interprets that as the endpoint coordinates and draws a horizontal line of 100 units long additional format: relative coordinates: `@200,100` Would be interpreted as offset coordinates to the last drawn point. eg: * select the line tool * enter `100,100` * the line tool interprets that as a click on coordinate 100,100 and takes that point as the start point of the line * enter `@300,0` * the line tool interprets that as the endpoint coordinates relative to the 100,100 point and draws a horizontal line of 300 units long