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: Ibc0918ad925066a7cbe0e8888edc4d3431d8c154
orig: a981fc78abdb4bb61fc76deb8d15016d63bc3da9
Signed-off-by: Shilpa Singh <shilpa.singh@samsung.com>