app: add interpreter for the GUM language

GUM is a small DSL, used in some property keys of GEGL operations
to dynamically control UI attributes based on context.  This commit
only adds an interpreter for the language; see the next commit for
the actual handling of the relevant keys.

See the comment at the top of gimppropgui-eval.c for a description
of the language.

Note that the interpreter is licensed under the LGPL.

app/widgets/Makefile.am

@@ -303,6 +303,8 @@ libappwidgets_a_sources = \
 	gimppropgui.h			\
 	gimppropgui-constructors.c	\
 	gimppropgui-constructors.h	\
+	gimppropgui-eval.c		\
+	gimppropgui-eval.h		\
 	gimppropwidgets.c		\
 	gimppropwidgets.h		\
 	gimpradioaction.c		\

app/widgets/gimppropgui-eval.h

@@ -0,0 +1,36 @@
+/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995-1997 Spencer Kimball and Peter Mattis
+ *
+ * gimppropgui.h
+ * Copyright (C) 2017 Ell
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __GIMP_PROP_GUI_EVAL_H__
+#define __GIMP_PROP_GUI_EVAL_H__
+
+
+gboolean   gimp_prop_eval_boolean (GObject     *config,
+                                   GParamSpec  *pspec,
+                                   const gchar *key,
+                                   gboolean     default_value);
+
+gchar    * gimp_prop_eval_string  (GObject     *config,
+                                   GParamSpec  *pspec,
+                                   const gchar *key,
+                                   const gchar *default_value);
+
+
+#endif /* __GIMP_PROP_GUI_EVAL_H__ */