Update screenshots

As part of the release of Gajim 1.0 the screenshots should be updated. If possible, the recommendation from https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#qsr-app-screenshots-info should be applied.

Note, if you change their filenames, don't forget to update the URLs in https://dev.gajim.org/gajim/gajim/blob/master/data/org.gajim.Gajim.appdata.xml.in