Fix WM_CLASS in desktop file

The WM_CLASS we set on all windows is just "vncviewer", so that it
matches the name of the .desktop file, which is what GNOME expects.

vncviewer/vncviewer.desktop.in.in

@@ -6,5 +6,5 @@ Exec=@CMAKE_INSTALL_FULL_BINDIR@/vncviewer
 Icon=tigervnc
 Terminal=false
 Type=Application
-StartupWMClass=TigerVNC Viewer: Connection Details
+StartupWMClass=vncviewer
 Categories=Network;RemoteAccess;