Allow filter_count on belts and Splitters

[Ranakastrasz]

The Filter Inserter's Filter mechanic should be supported by the Belt and Splitter entity type.
If set, you can set it's filter. Any item not matching will stop on reaching said belt, and not be teleported by the splitter.

[Rseding91]

Adding that kind of logic to belts and splitters would increase the RAM usage, save file size, and reduce game performance even when they aren't used by the belts or splitters.

So, that's almost guaranteed to never happen.

[Ranakastrasz]

In that case

Add a new entity prototype - Custom Belt. Allows input/output mapping, and filters. Has to be added by mod, only effects this one entity type.