"Lock tangency (T)" interface#67

Currently:
Locking Tangency (shortcut T) currently requires us to select an edge and its vertex at the end we want to lock.

Suggestion:
This can change so that it only requires us to select the edge. Which end is to be locked? We will just click closer to that end when we are selecting the edge.

What do you think? Would this help or confuse?

Completed, v. 1.8.0