unused function SH_Text_enlarge

The function SH_Text_enlarge was used as the common functionality of
SH_Text_append_string and SH_Text_append_text. It was first ported to
the new internal data structure, but then integrated into the function
SH_Text_append_string, because the function SH_Text_append_text doesn't
need it anymore and having the user calling it is now not as useful as
originally intended, both for the user and the library, so it shouldn't
be exposed. Also the method SH_Text_append_string is easier to
implement, if it is written as a single function.
Neithertheless it is commited here for future reference.