Implement global options as a property set

[uliska]

% Storage for all property sets
\registerOption #'(_propsets) #'()
\definePropertySet #'(OLL global) #'()

% Initialize option branch for oll-core
\registerOption oll-core.root #(this-parent)

% Create these nodes as oll-core is not loaded through \loadPackage
\registerOption loaded-packages #'(oll-core)
\registerOption loaded-modules.oll-core #'()

All these should be implemented as a property set.
Using a separate alist for #(_propsets) is not that simple.