The preset option was plugin_preference_overrides while the GUI field type
that renders it was GUIType::plugin_config, for one and the same thing.
Settle on "config": the store, the dialog and the Python hooks all say config
already, so renaming that way touches 4 files instead of the whole plugin
subsystem and the public plugin API.
Keep the _overrides suffix — the option is the preset's override layer over
the base PluginConfig store, a distinction EffectiveCapabilityConfig tracks.
Also wrap the printer tab's group heading in L(); it was the only one of the
three missing it, and was therefore untranslatable.
Ian Chua