Fix Flathub's recent appstream validation warnings

- fix missing captions on screenshots
- fix missing vcs-browser url

gnucash/gnome/gnucash.appdata.xml.in.in

@@ -41,12 +41,15 @@ ${REL_INFO}
   <screenshots>
     <screenshot type="default">
       <image type="source" width="1049" height="590">https://www.gnucash.org/images/features/feature_accounts_lg.png</image>
+      <caption>The account overview page is the starting point for managing your finances</caption>
     </screenshot>
     <screenshot>
       <image type="source" width="1049" height="590">https://www.gnucash.org/images/features/feature_register_lg.png</image>
+      <caption>The checkbook-style register provides a custom, convenient and familiar interface to entering financial transactions</caption>
     </screenshot>
     <screenshot>
       <image type="source" width="1049" height="590">https://www.gnucash.org/images/features/feature_bar_chart_lg.png</image>
+      <caption>A bar chart report showing a breakdown of expenses over time. This is one example of the many reports and graphs GnuCash provided out of the box.</caption>
     </screenshot>
   </screenshots>
   <update_contact>gnucash-devel@gnucash.org</update_contact>
@@ -57,4 +60,5 @@ ${REL_INFO}
   <url type="help">https://gnucash.org/docs.phtml</url>
   <url type="donation">https://gnucash.org/donate.phtml</url>
   <url type="translate">https://wiki.gnucash.org/wiki/Translation</url>
+  <url type="vcs-browser">https://github.com/Gnucash/gnucash</url>
 </component>