Let the New "No Item" Signal be Selected Directly from the GUI

[sczsne]

TL;DR

Allow the "No Item" (gray ) signal that appears in circuit-controlled splitters to be selected from the GUI when setting filters on splitters.

What?

When splitters got circuit control a few months ago, this introduced a new signal, "No Item." The only place this appears normally is in the "filter" slot of a splitter when a new splitter is connected to a wire, and the user selects the "filter" checkbox. Even if this signal cannot be found in the GUI anywhere else, it can be picked and placed into the hotbar. It can be set as the filter for non-circuit controlled splitters, disabling one of the outputs, effectively making the splitter work in a "merge only" mode.

Why?

It's common practice to use deconstruction planner as the filter item to block off one of the outputs of a splitter. Now that we have a signal that's explicitly supposed to be used to block one of the outputs on a splitter, I would like to be able to select this signal out of the filters UI instead of needing to steal it from a splitter.