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

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