Fix phantom point bug

STR for bug:
1. Load input/multiple_four_vertical_lines
2. Delete a node from the second vertical line.
3. Press 1 to switch to the SingleImploding algorithm.
4. Right-click on any spot in the upper area between the second and third line (to activate the eraser).
5. Observe that there's a line to a phantom point, i.e. a point that does not exist.

After this commit, observation 5 does not happen any more.
The bug was caused by an undocumented and unchecked precondition, namely
that all node IDs must start at 1, not 0.

Also refactored drawNode to drawPoint to avoid the use of fake IDs for
drawing the eraser.