@titlefont and strong Note warning in C plaintext/Info

* tta/C/convert/convert_to_plaintext.c
(plaintext_converter_initialize),
tta/C/main/plaintext_converter_state.h (PLAINTEXT_CONVERTER_STATE),
tta/perl/Texinfo/Convert/PlaintextNonXS.pm
(converter_initialize, _convert): add warn_strong_note field in
PLAINTEXT_CONVERTER_STATE and set based on converter format in
plaintext_converter_initialize.  In Perl, set warn_strong_note too in
converter_initialize instead of calling the function for each @strong.

* tta/C/convert/convert_to_plaintext.c
(convert_to_plaintext_internal): add warning for Note in @string.
Convert titlefont.