Update Appstream metadata

Fix all the validation warnings: Add a <developer> element, remove plain
text URLs from the description, and add captions to the screenshots. While
we're at it also add a link to Transifex.

com.inform7.IDE.appdata.xml

@@ -6,6 +6,9 @@
   <content_rating type="oars-1.0" />
   <name>Inform</name>
   <summary>Write interactive fiction</summary>
+  <developer id="com.inform7">
+    <name>Graham Nelson et al.</name>
+  </developer>
   <keywords>
     <keyword translate="no">IDE</keyword>
     <keyword>interactive fiction</keyword>
@@ -41,24 +44,19 @@
       for discussions). Lastly, Inform is continuously maintained and developed. All bug
       reports are examined and acted on (and the app will show you how to post them).
     </p>
-    <p>
-      The source code behind the Inform 7 design system itself is linked here:
-      https://github.com/ganelson/inform/
-    </p>
-    <p>
-      The source code behind this IDE is linked here:
-      https://github.com/ptomato/inform7-ide/
-    </p>
   </description>
   <launchable type="desktop-id">com.inform7.IDE.desktop</launchable>
   <screenshots>
     <screenshot type="default">
+      <caption>Writing with Inform: Source and Documentation</caption>
       <image>https://raw.githubusercontent.com/ptomato/inform7-ide/main/pixmaps/screenshots/source.png</image>
     </screenshot>
     <screenshot>
+      <caption>Testing tools: Skein and Transcript</caption>
       <image>https://raw.githubusercontent.com/ptomato/inform7-ide/main/pixmaps/screenshots/skein.png</image>
     </screenshot>
     <screenshot>
+      <caption>Welcome to Inform!</caption>
       <image>https://raw.githubusercontent.com/ptomato/inform7-ide/main/pixmaps/screenshots/welcome.png</image>
     </screenshot>
   </screenshots>
@@ -68,6 +66,7 @@
   <url type="donation">https://ganelson.github.io/inform-website/donate/</url>
   <url type="vcs-browser">https://github.com/ptomato/inform7-ide/</url>
   <url type="contribute">https://github.com/ganelson/inform/</url>
+  <url type="translate">https://explore.transifex.com/ptomato/inform7-ide/</url>
   <releases>
     <release version="2.0.0" date="2022-09-03">
       <description>