[pygtk] call set_label() periodically

[saeed]

>... and if it happens (usually to accomplish operations which take much
>time to execute and would freeze the GUI)

When a function takes a sensible time to execute, to prevent GUI from
freezing, we can do frequently(mostly in the loop that takes time):

while gtk.events_pending():
      gtk.main_iteration_do(False)

No need to manual multithreading only for this reason.