app/dock: fix "fix_reparent" caused embedded window not drawing.

Drawing of embedded windows rely on GdkWindows's composited feature.
In Gdk, the expose events of composited window will pass to its parent
window. The drawing of embedded windows rely on this mechanism.
So only use XReparentWindow doesn't satisfied. Simply invoke
gdk_window_reparent with the same parameters.

The "fix_reparent" was introduced in CL 1311.

Change-Id: I3e4e53fccfa340eca5b319f016c6df05719c266d