pdb: add a new PDB group "item" and move lots of functions to it

The item groups has all the duplicated functionality from drawable
and vectors (name, visible, linked etc).

Hijack the unused GIMP_PDB_REGION and turn it into GIMP_PDB_ITEM;
change all protocol aware files accordingly and bump the protocol
version number. Change script-fu to handle the new type.

libgimp/gimp.h

@@ -127,9 +127,10 @@ union _GimpParamData
   gchar           **d_stringarray;
   GimpRGB          *d_colorarray;
   GimpRGB           d_color;
-  GimpParamRegion   d_region;
+  GimpParamRegion   d_region; /* deprecated */
   gint32            d_display;
   gint32            d_image;
+  gint32            d_item;
   gint32            d_layer;
   gint32            d_layer_mask;
   gint32            d_channel;