Expose transport line spacing

[Therax]

See 63038. I would envision this as a startup only setting, possibly a "utility-constant". I'm hoping this is something that can be done, especially since in 0.16 transport lines can already be "overfilled" temporarily due to inserter and sideloading behavior.

[Bilka]

From what I can see, allowing this to be defined by mods would require changes that negatively impact the performance of all belts, so I am not going to add this.

[eradicator]

From what I can see, allowing this to be defined by mods would require changes that negatively impact the performance of all belts, so I am not going to add this.

Even if it was one setting for all belts, not a per-prototype value? (as suggested in the mentiond other thread)