Make machines have an editor location

[tatjam]

This will improve editor usability as machines will have a logical position within the piece. We may leave the standard centered "polygon" mode for machines which have no position.

Implementation:
Add a new entry to the Machine class which indicates a marker assigned to said machine. This may not be present. The editor then must properly position the piece.
Visibility could be implemented piece-wise or machine-wise, this requires some testing to find the most comfortable solution.

[tatjam]

Something to keep in mind too is that the machine must still be clearly linked to the part it's on. Maybe an highlighting effect will be enough, otherwise some kind of indicator may be needed

[tatjam]

After adding this it has become evident that the editor needs some kind of click masking. I believe the most efficient way would be to simply let the machine closest to the camera get the click.