Auto-lengthing and growing of tree edges

[MuTsunTsai]

This is proposed by Feelinghappy on Discord. It contains two ideas:

1. Automatically change the length of tree edges by their actual drawing length on the sheet. (For an edge of non-integral length, use the floor of it as the value).
2. Add a hot key so that when pressed, one can directly grow a new edge from an existing vertex by dragging the mouse. This would work especially well with the previous feature.