app, libgimp, pdb, plug-ins: add ability to default to context for all resource args.

Also fixes the passing of the resource param definitions through PDB.
There was some weird assumption, with a comment, in commit 73733335c8
that this was unneeded, which meant that we were not able to properly
recreate the right param spec over the wire.

plug-ins/python/palette-offset.py

@@ -58,7 +58,7 @@ class PaletteOffset (Gimp.PlugIn):
                                          GObject.ParamFlags.READWRITE)
             procedure.add_palette_argument ("palette", _("_Palette"),
                                             _("Palette"), True,
-                                            None, # Default to context.
+                                            None, True, # Default to context.
                                             GObject.ParamFlags.READWRITE)
             procedure.add_int_argument ("amount", _("O_ffset"), _("Offset"),
                                         1, GLib.MAXINT, 1, GObject.ParamFlags.READWRITE)