[gthumb] appdata: added project license, use SPDX strings

[Paolo Bacchilega <paobac src gnome org>]

commit 1894ee534e805afd6a11f2a16c5442b6281a8f13
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Fri May 2 19:28:02 2014 +0200

    appdata: added project license, use SPDX strings

 data/appdata/gthumb.appdata.xml.in |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/data/appdata/gthumb.appdata.xml.in b/data/appdata/gthumb.appdata.xml.in
index 126fb1e..1e6443e 100644
--- a/data/appdata/gthumb.appdata.xml.in
+++ b/data/appdata/gthumb.appdata.xml.in
@@ -2,7 +2,8 @@
 <!-- Copyright 2014 Paolo Bacchilega <paobac src gnome org> -->
 <application>
   <id type="desktop">gthumb.desktop</id>
-  <licence>CC0</licence>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-2.0+</project_license>
   <description>
     <_p>
       gThumb is an image viewer, editor, browser and organizer.  It is designed