Move std::cout calls to std::cerr and avoid printing ustrings

Leaves most instances from between DEBUG macros and commented out code alone.

std::cout calls should not be done for warnings/debugging, particularly if writing out a file to standard output too.

Glib::ustring conversions avoid throwing an exception. Same fix as 32d968cb, see g_iconv documentation https://docs.gtk.org/glib/func.iconv.html). Emit garbage data to terminal instead.

#1625 #3487

---

Should be 2 commits but I'm a bit short on Inkscape time and shouldn't affect review time.

Update made some modifications.

- Text toolbar change: Re-add error, but make it more descriptive.
  Keeping message to minimize number of changes that require thought.
- Prototype dialog change: can't compile the dialog, but std::cerr
  looks more appropriate here.
- remove unnecessary \n from g_warning