Create a new Gnome.DockItem widget containing widget, and add it
to app's dock with the specified layout information. Notice that,
if automatic layout configuration is enabled, the layout is
overridden by the saved configuration, if any.
item : Item to be added to app's dock
placement : Placement for the new dock item, one of Gnome.DockTop, Gnome.DockRight, Gnome.DockBottom, Gnome.DockLeft and Gnome.DockFloating
band_num : Number of the band where the dock item should be placed
band_position : Position of the new dock item in band band_num
offset : Offset from the previous dock item in the band; if there is no previous item, offset from the beginning of the band.