clipboard: Add gdk_clipboard_set()

clipboard: Add gdk_clipboard_set()

This API allows specifying a GType and va_args of a value of that type
to set the clipboard contents. This massively simplifies setting weird
object types into the clipboard.
2 example patches included in this patch are the GtkTextBuffer and the
file list in the file chooser.

Using gobject-introspection, this should work without specifying the
type, so that you can literlally say
  clipboard.set ("Hello World")
or
  clipboard.set (pixbuf)
which is why I've also marked all other setters as (skip). They just
exist in C as wrappers for type safety reasons.