Install enum header, and try to get role name from enum

Install enum header, and try to get role name from enum

Have atspi_accessible_get_role_name try to use the internal string associated
with the accessible's role rather than calling GetRoleName.  This avoids
making a DBus call when the role is cached.

Also, atspi.h now includes atspi-enum-types.h, and the latter is now installed.