Rename and move the methods to set flags to hildon-private.h

Rename and move the methods to set flags to hildon-private.h

* hildon/hildon-gtk.c: (do_set_progress_indicator),
(do_set_do_not_disturb), (do_set_portrait_flags): call
the new private method hildon_gtk_window_set_clear_window_flag()
(hildon_gtk_window_set_progress_indicator),
(hildon_gtk_window_set_do_not_disturb),
(hildon_gtk_window_take_screenshot): hildon_gtk_window_set_flag()
* hildon/hildon-private.c: (+hildon_gtk_window_set_clear_window_flag),
(+hildon_gtk_window_set_flag): Move from hildon-gtk.c and rename
properly.
* hildon/hildon-private.h: Add the two new private methds.