Add feature registry.
Integrate feature registry with modules.

In case a module is attached to a disabled feature, the module lookup
will give a warning message and ignore that (builtin) module.
Integrate feature registry with functions.

In case a function is attached to a disabled feature, the function lookup
will give a warning message and ignore that (builtin) function.
Add GUI Settings page to enable/disable features.
Handle synchronization between Qt settings / GUI and Feature objects.
Add concat() function to list of experimental features.