Get rid of the rest of variables exported from libgtk

Get rid of the rest of variables exported from libgtk

Especially the gtk_*_type ones in gtktexttypes.h were mentioned in
gtk.symbols presumably by accident. That header isn't even installed,
so no way can they be supposed to be public.

gtk_text_attr_appearance_type is from the installed but "semi-private"
gtktextlayout.h, so drop that one too from gtk.symbols for now.

The use of gtk_text_unknown_char_utf8 is bit of a mess. Code in a few
files knew implicitly that it is three bytes. Define a symbolic name
for the length of it instead. Add an exported function
gtk_text_unknown_char_utf8_gtk_tests_only() that returns a pointer to
it just for the sake of gtk/tests/textbuffer.c. Prefix the variable
with an underscore.

I doubt the usefulness of the test_utf8() in textbuffer.c. If it could
be dropped, gtk_text_unknown_char_utf8_gtk_tests_only() could be
dropped, too.