rename zim icon to "org.kiwix.desktop.x-zim"

flatpack doesn't export files that aren't prefixed like the app (org.kiwix.desktop)
so it has to be renamed and to be precised in the mimetype.xml file because it doesn't matched to the mimetype's name anymore

kiwix-desktop.pro

@@ -126,7 +126,7 @@ unix {
   INSTALLS += app_icon24 app_icon32 app_icon48 app_icon128 app_icon256
 
   mime_icon48.path = $$PREFIX/share/icons/hicolor/48x48/mimetypes
-  mime_icon48.files = resources/icons/kiwix/application-org.kiwix.desktop.x-zim.png
+  mime_icon48.files = resources/icons/48/kiwix/org.kiwix.desktop.x-zim.png
   INSTALLS += mime_icon48
 
   mime_file.path = $$PREFIX/share/mime/packages/

resources/org.kiwix.desktop-mime.xml

@@ -7,6 +7,7 @@
          </magic>
          <glob pattern="*.zim"/>
          <glob pattern="*.zimaa"/>
+         <icon name="org.kiwix.desktop.x-zim"/>
   </mime-type>
 </mime-info>