libgimp: new metadata support concept in GimpSaveProcedure and new…

… class GimpSaveProcedureDialog.
The idea is that we have basically the same code in most file format
plug-ins to handle various generic metadata, yet usually with slight
differences here and there. Even behavior is sometimes a bit different
even though there is no reason for the logics to be different from one
format to another.

So I move the metadata support logics into GimpSaveProcedure (and
GimpProcedureConfig still keeps the main export logics). The GUI logics
is now in a new GimpSaveProcedureDialog. So export plug-ins will now get
the creation of generic metadata nearly for free. All they have to do is
to tell what kind of metadata the GimpSaveProcedure supports with the
gimp_save_procedure_set_support_*() functions.

Then consistency will apply:
- If a format supports a given metadata, they will always have an
  auxiliary argument with the same name across plug-ins.
- The label and tooltips will also be always the same in the GUI.
- Order of metadata widgets will also stay consistent.
- The widgets will work the same (no more "Comment" text view missing in
  one plug-in but present in another, or with an entry here, and a text
  view there, and so on).

Also adding gimp_save_procedure_dialog_add_metadata() to allow plug-ins
to "declare" one of their options as a metadata option, and therefore
have it packed within the "Metadata" block which is now created (for
instance for PNG/TIFF/JPEG specific metadata). This allows a nicer
organization of dialogs.

libgimp/gimpui.h

@@ -43,6 +43,7 @@
 #include <libgimp/gimpproceduredialog.h>
 #include <libgimp/gimpprocview.h>
 #include <libgimp/gimpprogressbar.h>
+#include <libgimp/gimpsaveproceduredialog.h>
 #include <libgimp/gimpselectbutton.h>
 #include <libgimp/gimpzoompreview.h>