Allow pairs of symbols as Signals in Signal Selector

[redrook]

In Signal Selector in any context, allow (optionally) to specify a pair of symbols as a signal. It could be thought of as a symbol with a subscript. For example, signals could be [oil] or [1], [steam] or [E] (currently available), or [oil_1] and [steam_E] (new) if seen as subscripted, or ([oil], [1]) and ([steam], [E]) if seen as a pair.

Subscripted view highlights that the order matters (which symbol is the first element of the pair, and which is second).

These new symbols don't need to be treated specially in any way by existing logical items (such as summing up done implicitly by both red and green wires - only identical symbols get summed up, sorting functions, and so on) - if anything special needs to be done, it's on user to aggregate and/or analyze.

The motivational applications are simple: distinguish in the same network "amount of steam stored" and "minimum value of the amount of steam stored", where "minimum" modifier is common and applies to "accumulator charge", "oil", "petroleum gas" and so on, making workarounds like "M" or "arrow down" impractical.

The other example is tracking storage at different mining outposts: "oil stored at outpost Oil 1", "oil stored at outpost Oil 2" and so on.