Update appstream file (#775)

Use RDNS id.
Replace deprecated 'developer-name' with 'developer' section.
Remove non standard 'kudos' section.

data/rednotebook.appdata.xml

@@ -1,12 +1,14 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <component type="desktop">
   <!-- https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html#tag-id-desktopapp -->
-  <id>rednotebook</id>
+  <id>app.rednotebook.rednotebook</id>
   <metadata_license>CC0-1.0</metadata_license>
   <!-- https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-project_license -->
   <project_license>GPL-2.0+</project_license>
   <name>RedNotebook</name>
-  <developer_name>Jendrik Seipp</developer_name>
+  <developer>
+    <name>Jendrik Seipp</name>
+  </developer>
   <update_contact>jendrikseipp_AT_gmail.com</update_contact>
   <summary>Graphical diary and journal</summary>
   <description>
@@ -37,12 +39,6 @@
       <image>https://rednotebook.app/screenshots/rednotebook-2.0-2.png</image>
     </screenshot>
   </screenshots>
-  <kudos>
-    <kudo>ModernToolkit</kudo>
-    <kudo>HiDpiIcon</kudo>
-    <kudo>HighContrast</kudo>
-    <kudo>UserDocs</kudo>
-  </kudos>
 
   <!-- https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-content_rating -->
   <content_rating type="oars-1.0" />