res:dock:: fix trayicons position problem.

the offsetTop is the distance of the current element relative to the top of the
offsetParent node. so $("#system").offsetTo does not work.

using $("#panel").offsetTop is ok, $("#panel")'s offsetParent is body. if
screen height is changed, $("#panel").offsetTop is always changed.

Change-Id: Icec787dc46bdff59171615eefba6ec210971e3ed