Performance improvement in getting accessible active window.

Performance improvement in getting accessible active window.

First we try to deteremine PID of currently active app using
ecore_x_window_focus_get. Then we iterate over accessible child apps
filtering by that app pid. If something goes wrong we fall back to previous
algorithm. Additionally some memory management fixes were added.
This improvement is proposed because iterating over accessible app windows
when app is not active is very time consuming (non active aps have low priority
and every dbus call takes a few hundred ms).

Change-Id: Ia18401200afa2bbc5f06e511f440f0fb51b16cd4