Add atspi_accessible_get_default_label_info

Add atspi_accessible_get_default_label_info

It is not possible to support default label feature using current at-spi2-core
APIs without much of IPC. The following would be difficult case to cover.

(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.

The atspi_accessible_get_default_label_info returns default label information
to be used screen-reader side. This is not stable. And this depends on toolkit
side UI definition. The candidate of default label object could be changed by
UI definition.

AtspiAccessibleDefaultLabelInfo *dli;
dli = atspi_accessible_get_default_label_info(root, &error);

You have to free alocated resource as below, if it is not necessary any more.
g_object_unref(dli->obj);
free(dli);

Change-Id: I444906e6d020ea6fe2536a73473e0e796a015d05