metainfo: Add screenshots

Fixes #341

data/org.gnome.Calls.metainfo.xml

@@ -18,6 +18,19 @@
     <developer_name>Evangelos Ribeiro Tzaras</developer_name>
     <developer_name>Julian Sparber</developer_name>
 
+    <screenshots>
+      <screenshot type="default">
+        <!-- Translators: A screenshot description. -->
+        <caption>Placing a call</caption>
+        <image type="source" width="720" height="1440">https://gitlab.gnome.org/GNOME/calls/raw/master/data/screenshots/calling.png</image>
+      </screenshot>
+      <screenshot>
+        <!-- Translators: A screenshot description. -->
+        <caption>The call history</caption>
+        <image type="source" width="720" height="1440">https://gitlab.gnome.org/GNOME/calls/raw/master/data/screenshots/history.png</image>
+      </screenshot>
+    </screenshots>
+
     <recommends>
         <control>pointing</control>
         <control>keyboard</control>