New circuit signal: "Read self"

[Super Mikal]

TL;DR

Add a new circuit signal option to entities with inventories (or possibly everything) that emits a value of 1 for itself, i.e. a Steel Chest would emit a Steel Chest value of 1.

What?

The simplest implementation would be just a checkbox for something like "Read self" with a tooltip, but it's also possible to imagine more complex options like an additional checkbox to disregard quality making it always emit normal quality.

Why?

It would save having to build and maintain a constant combinator or internal number of an arithmetic combinator when doing things like computing average chest contents at a train stop.

[Tertius]

Would be a very useful option, with it it's possible to just know how many tiles were built of some blueprint, and provide enough supply for it without having to manually define how many of these are currently present.

Which items? Probably just ones that can already be connected to the circuit network.

- chests
- all manufacturing/crafting machines
- recycler
- reactors
- storage tank
- roboports (they already provide some info, albeit in a different manner and not configurable)