autoWorkspaces: fix creation of new workspaces with application launchers

In the workspace-collecting code we add a check to avoid collecting a
workspace if any startup sequence is running there. Since the sequence
can take some time to load, an helper function is also added which keeps
the (empty) workspace around for a very short time, while waiting for the
sequence to start.

https://bugzilla.gnome.org/show_bug.cgi?id=664202