xgettext: Scheme: Prepare for recognizing more escape sequences in string literals.
* gettext-tools/src/x-scheme.c (accumulate_escaped): New function, extracted
from read_object.
(struct object): Add field 'mixed_string'.
(free_object): Free field 'mixed_string'.
(string_of_object): Implement differently for objects of type t_string.
(read_object): Use accumulate_escaped. Invoke mixed_string_alloc_utf8 instead of
mixed_string_alloc_simple.
