appDisplay: Check instanceof AppIcon using constructor inside AppIcon
It seems like some recent change broke the check whether the drag source is an instance of AppIcon. While the drag source indeed is an AppIcon and everything else works correctly, the check still returns false. Theoretically it makes sense that this doesn't work, because we're assigning AppIcon using `var AppIcon =` and that will only get set after `GObject.register_class()` finished, so accessing `AppIcon` inside that function seems risky and is probably wrong, but that doesn't really explain why it suddenly broke. Anyway, fix it by comparing to `this.constructor` instead of `AppIcon`, which works and we know for sure exists at this point.