Add appdata file

Applications wishing to have long descriptions, screenshots and other useful things
are required to ship one or more files in /usr/share/appdata/%{id}.appdata.xml.
see pttp://people.freedesktop.org/~hughsient/appdata/
Make the intltool take care of appdata file

The synfigstudio.appdata.xml.in is ready, but the content in it should be
improved if needed.
Add a new screenshot of 0.64.0 for using in Appdata.xml

Add a new screenshot accroding to the spec of appdata, 16:9 (1360x768)
Fix localization for appdata