[Dist] set the correct WM Class in the flatpak Desktop Entry (#4181)

* [Dist] set the correct WM Class in the flatpak Desktop Entry

* [Dist] Set `SingleMainWindow` in the flatpak Desktop Entry

This hints to the DE that the application only supports one "main"
window.

In GNOME, this prevents the DE adding a useless "New Window" button.

flatpak/com.heroicgameslauncher.hgl.desktop

@@ -2,9 +2,10 @@
 Name=Heroic Games Launcher
 Exec=heroic-run --ozone-platform-hint=auto %u
 Terminal=false
+SingleMainWindow=true
 Type=Application
 Icon=com.heroicgameslauncher.hgl
-StartupWMClass=Heroic
+StartupWMClass=heroic
 Comment=An Open Source Launcher for GOG, Epic Games and Amazon Games
 Comment[de]=Ein OSS-Spielelauncher für GOG, Epic Games und Amazon Games
 Comment[pl]=Otwartoźródłowy launcher dla GOG, Epic Games i Amazon Games