Reimplement pixel measurement tool

[yeahitsjan]

The current state of the pixel measurement tool ist not satisfying. I would like to reimplement and complete some ideas here. The best idea I had until now is the following (image is not mine, but edited):

The label showing the current distance should be getting a rectangle around it. The user selects two points in the scene after choosing the pixel measurement tool and then a line is drawn onto the scene. We could make this resizable via the resizer-item.