| Generic TI varname to filename transcoding

### ticonv_utf16_to_gfe

TIEXPORT4 char* TICALL ticonv_utf16_to_gfe(CalcModel model, const unsigned short *src)

This function converts a varname into a valid filename (depends on locale). Example: 'foobar' => foobar, 'alpha' => _alpha_.

model : | a calculator model taken in #CalcModel.
src : | the name of variable to convert from UTF-16
Return value : | %dst as a newly allocated string.

### ticonv_gfe_to_zfe

TIEXPORT4 char* TICALL ticonv_gfe_to_zfe(CalcModel model, const char *src_)

This function converts a locale dependent filename into a valid ZIP filename. Example: 'foobar' => foobar, 'alpha' => _alpha_.

model : | a calculator model taken in #CalcModel.
src : | the name of variable to convert from GLib filename encoding
Return value : | %dst as a newly allocated string.

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