Extract rxThreshold into JS8::Submode. This function was a slot, but was never used as one.
Extract rxSNRThreshold into JS::Submode. Note that this function was a slot, but was never used as such.
Consolidates the majority of JS8 submode constant data and submode-specific computatation functions into a single location. These are at present very spread out in the MainWindow class, with some code duplication in the plotter.