software: Added missing screenshots in software

Updated appdata.xml

Fixes #588

shell/appdata/gnome-control-center.appdata.xml.in

@@ -11,6 +11,24 @@
       Settings is the primary interface for configuring your system.
     </p>
   </description>
+  <screenshots>
+    <screenshot type="default">
+      <image type="source" width="1920" height="1048">https://gitlab.gnome.org/GNOME/gnome-control-center/raw/master/shell/appdata/GNOME_Settings_Sound_Panel.png</image>
+      <caption>GNOME Settings Sound Panel</caption>
+    </screenshot>
+    <screenshot>
+      <image type="source" width="1920" height="1048">https://gitlab.gnome.org/GNOME/gnome-control-center/raw/master/shell/appdata/GNOME_Settings_Mouse_Panel.png</image>
+      <caption>GNOME Settings Mouse &amp; Touchpad Panel</caption>
+    </screenshot>
+    <screenshot>
+      <image type="source" width="1920" height="1048">https://gitlab.gnome.org/GNOME/gnome-control-center/raw/master/shell/appdata/GNOME_Settings_Background_Panel.png</image>
+      <caption>GNOME Settings Background Panel</caption>
+    </screenshot>
+    <screenshot>
+      <image type="source" width="1920" height="1048">https://gitlab.gnome.org/GNOME/gnome-control-center/raw/master/shell/appdata/GNOME_Settings_Keyboard_Panel.png</image>
+      <caption>GNOME Settings Keyboard Panel</caption>
+    </screenshot>
+  </screenshots>
   <url type="bugtracker">https://gitlab.gnome.org/GNOME/gnome-control-center/issues/</url>
   <url type="donation">https://www.gnome.org/friends/</url>
   <url type="translate">https://wiki.gnome.org/TranslationProject</url>