atspi: Handle default label object on toolkit side

atspi: Handle default label object on toolkit side

There are unexpected difficult cases to make default label work correctly.
The following would be the case.

(top of accessible tree) - (bottom side)
PageTab1 - Panel1 - PageTab2 - Panel2  -  PageTab3 - Panel3(currently showing)

Application could make as below:
PageTab1 - Panel1 - PageTab3 - Panel3  -  PageTab2 - Panel2(currently showing)

or following tree would normally be possilbe:
PageTab1 - Panel1 - PageTab2 - Panel2(currently showing)

There are much of complicated case over there.
So we are handling the default label object stack on toolkit(Elementary) side.

Please refer to following patch set:
https://review.tizen.org/gerrit/#/c/151025/ (at-spi2-core)
https://review.tizen.org/gerrit/#/c/151030/ (screen-reader)

Change-Id: I0834f9d0625f4e787d6315ce4858bdec4a67ab18