workspace: Use AppIcon.app to check action-support by the drag source

`AppIcon.shellWorkspaceLaunch()` can easily be replaced by checking for
`AppIcon.app` and calling `AppIcon.app.open_new_window()` directly.

For compatibility and to prevent breaking extensions implementing the
function, keep supporting the `shellWorkspaceLaunch` API in AppIcon
while logging a deprecation warning. Also keep supporting the API on
drag sources (without deprecating it) to allow extensions to define
custom actions on their drag sources.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/121

js/ui/appDisplay.js

@@ -2222,6 +2222,9 @@ var AppIcon = class AppIcon {
     }
 
     shellWorkspaceLaunch(params) {
+        let { stack } = new Error();
+        log(`shellWorkspaceLaunch is deprecated, use app.open_new_window() instead\n${stack}`);
+
         params = Params.parse(params, { workspace: -1,
                                         timestamp: 0 });