[gimp] desktop: fix duplication of the app icon in a dock like Plank

[Jehan <jehanp src gnome org>]

commit 1c2d1003e55fce527c3dd3634fd784214178797c
Author: Stan Janssen <stan finetuned nl>
Date:   Thu Aug 12 18:33:47 2021 +0200

    desktop: fix duplication of the app icon in a dock like Plank
    
    Without this entry, when starting GIMP as a flatpak application on
    elementaryOS, the Dock icon would get duplicated, as if the application
    was detached from its launcher. This entry fixes that by allowing the
    dock to associate the app's window with the desktop entry.
    
    More information on the type of problem this caused can be found here:
    https://github.com/elementary/dock/issues/119

 desktop/gimp.desktop.in.in | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/desktop/gimp.desktop.in.in b/desktop/gimp.desktop.in.in
index c0a98e9c6c..66f5b355c1 100644
--- a/desktop/gimp.desktop.in.in
+++ b/desktop/gimp.desktop.in.in
@@ -13,3 +13,4 @@ Terminal=false
 Categories=Graphics;2DGraphics;RasterGraphics;GTK;
 StartupNotify=true
 MimeType=@MIME_TYPES@
+StartupWMClass=gimp-@GIMP_APP_VERSION@