DISTS: Set StartupWMClass in .desktop file

- due to the change to the rDNS-based filename some window managers like
  Cinnamon can't find the according desktop file for scummvm's
  application window anymore
- this causes some unwanted behavior (e.g. not using the correct icons
  in the task switcher)
- before commit 4955800 the mapping
  worked by matching the WM_CLASS property of the scummvm window to
  the filename of the desktop file
- this patch sets the StartupWMClass property in the .desktop file to
  the name of scummvm's WM_CLASS X property which will allow a proper
  mapping

dists/org.scummvm.scummvm.desktop

@@ -13,3 +13,4 @@ Terminal=false
 Type=Application
 Categories=Game;AdventureGame;
 StartupNotify=false
+StartupWMClass=scummvm