Revert "res:dock:: fix preview window problem."

This reverts commit eeaad5e3776ac6d5e20d7d58cf93c2ed5d18f05c.

会导致dde-dock-applets无法使用，鼠标放上去里面就消失了

Change-Id: I1372a1542e7c7cf2bef96f6992e4d22fe7ade9d4