Eliminate a use of GetGlobalConfig().

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

For easier state management inside filter plugins.

The old struct config_param remains only for top-level string options.

Merge duplicate code.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.