Add implementation of atk_attribute_register, atk_attribute_for_name Udate

Add implementation of atk_attribute_register, atk_attribute_for_name Udate

* atk/atktext.c:
Add implementation of atk_attribute_register, atk_attribute_for_name
Udate atk_attribute_get_name to use ATK_TYPE_TEXT__ATTRIBUTE and
support extra attributes being defined

* atk/atktext.h:
Add ATK_TEXT_ATTR_INVALID and ATK_TEXT_ATTR_LAST_DEFINED to allow
extra attributes to be defined
Add atk_attribute_register() and atk_attribute_for_name()

* atk/atk.def:
Add new functions

* docs/atk-sections.txt docs/tmpl/atktext.sgml
Update because of additions to atk/atktext.h

* tests/testrelation.c:
Add tests for text attributes