Allow custom robot docking point for logistic-container entities

[dgw]

The new logistic chest design used in 0.17 has a port on top that animates open when a robot accesses the chest. Roboports have a port on top that robots enter and leave through (presumably its position relative to the entity center is given by the stationing_offset property).

Designing custom logistic containers (like warehouses, which is the application Anoyomouse and I have in mind) would be a lot more interesting if robots were able to pick up and drop off items at somewhere besides {0, 0}, so the animated portion of the entity doesn't have to be in the center.

[Bilka]

Okay, added for the next version.

[dgw]

Awesome, thanks so much!