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

src/shell-window-tracker.c

@@ -816,6 +816,12 @@ shell_startup_sequence_get_completed (ShellStartupSequence *sequence)
   return sn_startup_sequence_get_completed ((SnStartupSequence*)sequence);
 }
 
+int
+shell_startup_sequence_get_workspace (ShellStartupSequence *sequence)
+{
+  return sn_startup_sequence_get_workspace ((SnStartupSequence*)sequence);
+}
+
 /**
  * shell_startup_sequence_create_icon:
  * @sequence: