It was only used in one place. Use "g_base64_encode" instead, like the
rest of the codebase.

None of these functions seems to expect an actual gunichar (which is
a 32-bit type and can hold any UTF-32 or UCS-4 character code, also
known as a Unicode code point).

Instead we want UTF-8 encoded character data (i.e. gchar, which is
equivalent to char) that can be output byte-wise to form a valid
UTF-8 encoded string.

Found using `codespell -q 3 -w --skip="*.svg,*.po,*.ts,./share/tutorials,./src/libavoid,./packaging/win32/languages,./man,./src/2geom" -I ../inkscape-whitelist.txt`
whereby whitelist file contained:
```
dum
iff
glight
substract
te
upto
```

(bzr r12168)

(bzr r12125)

(bzr r12118)

(bzr r10746)

(bzr r10697)

(bzr r5594)

(bzr r1)