Add .DirIcon to AppImage (#6715)

* Add .DirIcon to AppImage
* Add X-AppImage-Version to .desktop file
* Closes #6695

cmake/linux/package_linux.sh.in

@@ -149,6 +149,7 @@ fi
 
 # Patch the desktop file
 sed -i 's/.*Exec=.*/Exec=lmms.real/' "$DESKTOPFILE"
+echo "X-AppImage-Version=@VERSION@" >> "$DESKTOPFILE"
 
 # Fix linking for soft-linked plugins
 for file in "${APPDIR}usr/lib/lmms/"*.so; do
@@ -201,6 +202,9 @@ fi
 rm -f "${APPDIR}/AppRun"
 ln -sr "${APPDIR}/usr/bin/lmms" "${APPDIR}/AppRun"
 
+# Add icon
+ln -srf "${APPDIR}/lmms.png" "${APPDIR}/.DirIcon"
+
 # Create AppImage
 echo -e "\nFinishing the AppImage..."
 run_and_log "$APPIMAGETOOL" "${APPDIR}" "@APPIMAGE_FILE@"