[TASK] Minor CGL changes to Core class
[TASK] Add a few missing API methods expected to exist by convention
[TASK] Avoid expensive try/catch, replace with isset() check
[TASK] Remove registration (not configuration) of Flux API plugin

This allows the plugin to be used in manual bootstrapping, but not inserted as plugin on pages.
[TASK] Adapt a few tests to recent changes
[TASK] Some leftovers from namespace change