Create mate-screenshot.appdata.xml

An appdata file for inclusion in the upcoming software centers as per the new freedesktop.org specs.

It should be placed in /usr/share/appdata/ similar to the way .desktop files are placed in /usr/share/applications/, e.g. if you have a "$(datadir)/applications" definition in your makefiles, you need to add a "$(datadir)/appdata" as well.

Please, skim through the file in case I made a mistake and please, include it in the 1.8 branch as well.

Thanks!