appDisplay: Check instanceof AppIcon using constructor inside the class

It seems like some recent change (maybe the move to a ClutterActor subclass for AppIcon) 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, which breaks the creation of new folders using DnD.

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.

Fix this by comparing to this.constructor instead of AppIcon, which works fine and we know for sure exists at this point.