Originally, Layer contained a std::map of Sections, which containted a std::map
containing the (key, value) pairs. Here we flattern this structure so that only
one std::map is required, reducing the number of indirections required and
vastly simplifying the code.
Not really used anywhere yet, but useful for not having to duplicate
config locations and for getting rid of conflicts when I get around
to rebase my Main.Core and Main.DSP porting PR.
