atspi: support "default_label" attribute of window

atspi: support "default_label" attribute of window

Wearable profile application does not want to use default label functionality
because it is hard to meet application UI definition.
So if the window has attribute "default_label" key, and its "disabled" value,
then screen-reader does not read out default label, and does not clear
currently highlight object.

Application should append attribute as below.

elm_atspi_accessible_attribute_append(window, "default_label", "disabled");

This patch set works with following patch set.

https://review.tizen.org/gerrit/151570 (screen-reader)

Change-Id: I354c6ecf30db3a0011658d69577ad384fcb900db