appdata: Improve appdata for AppStream 1.0

- Add the `<developer><name>` tag
- Mark the `<developer_name>` as deprecated
- Use appstreamcli to validate appdata
- Remove one of Purism tag to pass appstreamcli validation

data/org.gnome.Loupe.metainfo.xml.in.in

@@ -4,7 +4,11 @@
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-3.0-or-later</project_license>
   <name>Image Viewer</name>
+  <!-- developer_name tag deprecated with Appstream 1.0 -->
   <developer_name>The Loupe Team</developer_name>
+  <developer id="gnome.org">
+    <name>The Loupe Team</name>
+  </developer>
   <summary>View images</summary>
   <description>
     <p>Browse through your images and inspect their metadata with:</p>
@@ -54,7 +58,6 @@
   <content_rating type="oars-1.1" />
   <custom>
     <value key="GnomeSoftware::key-colors">[(208, 105, 167)]</value>
-    <value key="Purism::form_factor">workstation</value>
     <value key="Purism::form_factor">mobile</value>
   </custom>