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.
Set the border relief style of close button of canvas window tab to None
Add two new icons

Add show and snap guide line icons
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)
Show icon only for panel toolbar on all platforms