Speed up labeling tool

[jankrepl]

It seems like on certain machines there is a lag when adding keypoints. It would be great to

1. Profile the current implementation
2. Remove potential bottlenecks

[Stannislav]

Like mentioned elsewhere, maybe it's worth testing the impact of replacing

self.fig.canvas.draw_idle()

by

self.fig.canvas.draw()

in new_GUI.py.

I have vague memories of there being issues with the latter (GUI not always updated?) but I'm not so sure