doc: improve ATK_TEXT_ATTR documentation
state: add ATK_STATE_CHECKABLE and ATK_STATE_HAS_POPUP

https://bugzilla.gnome.org/show_bug.cgi?id=712576
https://bugzilla.gnome.org/show_bug.cgi?id=712581
roles: new ATK roles for ARIA roles (plus rating)

https://bugzilla.gnome.org/show_bug.cgi?id=712586