UI: Grease Pencil Modifier Drag and Drop, Layout Changes

This patch implements the list panel system D7490 for grease pencil
modifiers. It also moves their drawing to a callback in
GpencilModifierTypeInfo in line with the extensible architecture
refactoring goal T75724.

This also adds the "set_error" function for grease pencil modifiers,
which hadn't been copied from mesh modifiers yet.

The implementation is basically exactly the same as for the modifier
patch (9b099c8612).

Thanks to Matias Mendiola (mendio) for providing mockups for many
of the layout changes.

Differential Revision: https://developer.blender.org/D7978

source/blender/editors/include/UI_interface.h

@@ -2004,6 +2004,7 @@ void uiTemplatePathBuilder(uiLayout *layout,
                            struct PointerRNA *root_ptr,
                            const char *text);
 void uiTemplateModifiers(uiLayout *layout, struct bContext *C);
+void uiTemplateGpencilModifiers(uiLayout *layout, struct bContext *C);
 void uiTemplateConstraints(uiLayout *layout, struct bContext *C, bool use_bone_constraints);
 
 uiLayout *uiTemplateGpencilModifier(uiLayout *layout, struct bContext *C, struct PointerRNA *ptr);