[ATSPI] Sort children using middle line

[ATSPI] Sort children using middle line

So far, it has been judged that thye are on different lines
if the "y" value of child A is less than "y + 0.25 * h" value of child B.
But it caused unwanted children order in following case.

 [(class=ContentPage)],[0,0,720,1280]
     [(class=AppBar)],[0,0,720,120]
         [(class=Button)],[64,36,48,48]
         [(class=TextLabel)],[136,0,300,120],[NUI Tizen Gallery]
         [(class=Control)],[476,0,180,120]

The order should be "Button" > "TextLabel".

Change-Id: Icb11cf1fb63de7a5cea8fbf7697cabdf5a5179eb