Issue #14233: incorrect StartupWMClass in gimp.desktop.

The WM_CLASS X property is apparently set to windows throughout the
program by the g_set_prgname() call which we changed in commit
95e32ebb34 from being the executable name (being the fully qualified
program name with the version) to the desktop name.

desktop/gimp.desktop.in.in

@@ -18,4 +18,4 @@ Terminal=false
 Categories=Graphics;2DGraphics;RasterGraphics;GTK;
 StartupNotify=true
 MimeType=@MIME_TYPES@
-StartupWMClass=gimp-@GIMP_APP_VERSION@
+StartupWMClass=@GIMP_DESKTOP_NAME@

desktop/meson.build

@@ -4,6 +4,7 @@ desktop_conf = configuration_data()
 desktop_conf.set('GIMP_COMMAND',      gimp_command)
 desktop_conf.set('GIMP_VERSION',      gimp_version)
 desktop_conf.set('GIMP_APP_VERSION',  gimp_app_version)
+desktop_conf.set('GIMP_DESKTOP_NAME', gimp_desktop_name)
 desktop_conf.set('MIME_TYPES',        ';'.join(MIMEtypes))
 desktop_conf.set('BUG_REPORT_URL',    bug_report_url)