GDesktopAppInfo: add an accessor for StartupWMClass

Components using GIO to do window to application matching can
use that field to retrieve potential candidates.

https://bugzilla.gnome.org/show_bug.cgi?id=673659

gio/tests/appinfo-test.desktop

@@ -8,3 +8,5 @@ Comment=GAppInfo example
 Comment[de]=GAppInfo Beispiel
 Exec=./appinfo-test --option
 Icon=testicon
+StartupNotify=true
+StartupWMClass=appinfo-class