Plug-ins that work from different bindings probably want to use their
own list-type to specify arguments, rather than working with a more
cumbersome `GimpValueArray`.
This new API should make it less verbose. For example:
```
args = Gimp.ValueArray.new(5)
args.insert(0, GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE))
args.insert(1, GObject.Value(Gimp.Image, image))
args.insert(2, GObject.Value(Gimp.Drawable, mask))
args.insert(3, GObject.Value(GObject.TYPE_INT, int(time.time())))
args.insert(4, GObject.Value(GObject.TYPE_DOUBLE, turbulence))
Gimp.get_pdb().run_procedure('plug-in-plasma', args)
```
becomes
```
Gimp.get_pdb().run_procedure('plug-in-plasma', [
GObject.Value(Gimp.RunMode, Gimp.RunMode.NONINTERACTIVE),
GObject.Value(Gimp.Image, image),
GObject.Value(Gimp.Drawable, mask),
GObject.Value(GObject.TYPE_INT, int(time.time())),
GObject.Value(GObject.TYPE_DOUBLE, turbulence),
])
```
It's just too weird to be public. Remove its properties from the wire
protocol and from pluginrc. Instead, have all GParamSpecs' flags on
the wire and in pluginrc, so we can use stuff like
GIMP_PARAM_NO_VALIDATE.
Port the remaining few places to GIMP_PROC_ARG_STRING().
I'm sure something is broken now wrt UTF-8 validation,
will add tighter checks in the next commit.
and make them build again without any wrong symbols.
This does not enable the devel-docs build yet, I'll fix
module by module before enabling the build again.
