Fix desktop id and add screenshot captions

com.super_productivity.SuperProductivity.metainfo.xml

@@ -8,7 +8,7 @@
   <url type="homepage">https://super-productivity.com/</url>
   <url type="vcs-browser">https://github.com/johannesjo/super-productivity/</url>
   <url type="donation">https://github.com/sponsors/johannesjo</url>
-  <launchable type="desktop-id">com.super_productivity.SuperProductivity</launchable>
+  <launchable type="desktop-id">com.super_productivity.SuperProductivity.desktop</launchable>
   <icon type="stock">com.super_productivity.SuperProductivity</icon>
   <categories>
     <category>Office</category>
@@ -36,18 +36,23 @@
   <screenshots>
     <screenshot type="default">
       <image>https://super-productivity.com/static/7d2933a034590b3f40301a65e1d52538/34ae2/standard.png</image>
+      <caption>An overview of today's tasks, including time estimates and progress tracking</caption>
     </screenshot>
     <screenshot>
       <image>https://super-productivity.com/static/80c1ca336f5d3ad06390fdfecc7c6ad7/34ae2/jira.png</image>
+      <caption>Task details imported from GitHub</caption>
     </screenshot>
     <screenshot>
       <image>https://super-productivity.com/static/f259c271688b81febdf8082b22645823/34ae2/break-reminder.png</image>
+      <caption>Break reminder</caption>
     </screenshot>
     <screenshot>
       <image>https://super-productivity.com/static/d56fca55ab08d2464fb7b0bbd15d43a0/34ae2/timeline-calendar.png</image>
+      <caption>A timeline view of tasks, including integration with calendar events</caption>
     </screenshot>
     <screenshot>
       <image>https://super-productivity.com/static/92c8d60be9f55a4bf6a8ebc97e3bb6a5/34ae2/worklog.png</image>
+      <caption>Worklog view, showing a summary of tasks broken down by year, month, week, and day</caption>
     </screenshot>
   </screenshots>
   <releases>