Implement Supply, Fluid, and Buffer Depot Priority

[Bloodyaugust]

Using the network writer, I think I was able to implement workable priority setting for the various types of supply depot. See commit messages for detail, but basically, you send an amount on signal "0" to a circuit writer attached to a depot, and that becomes the priority for the depot. Higher priority depots get pulled from first by requesters and buffers. Normal heuristic applies to depots with the same priority. PR also adds a tip screen to explain, although only translated to English.

PS: This is my first delve into Factorio modding, and I haven't touched Lua in years... let me know if there are code quality concerns.