Add locale independent and thread safe string conversion API with tests

Add locale independent and thread safe string conversion API with tests

All conversions are performed as if in the "C" locale but without actually
changing locale.

This is a wrapper around printf/sscanf for int types which aren't affected by
locale and uses glib functions g_ascii_strtod and g_ascii_dtostr for
float/double types.

My first attempt at this used std::stringstream and
ios::imbue(std::locale::classic()) as it should be thread safe, but testing
shows it is not for gcc/mingw-w64 on Windows, and possibly also some versions
of macOS/OS X.

Use "yes" and "no" when converting a boolean in PBD::string_to<bool> as this
seems to be the convention used throughout libardour which will allow using
string_to<bool> in those cases.

Add accepted bool string values from PBD::string_is_affirmative to
PBD::string_to<bool>

Mark strings in pbd/string_convert.cc as not for translation

Add u/int16_t string conversions to pbd/string_convert.h and tests

Add DEBUG_TRACE output on conversion errors

Add int8_t/uint8_t conversions(using int16/uint16 types) to string_convert.h

Add support for converting an infinity expression to/from string

Follows the C99/C11 standard for strtof/strtod where subject sequence is an
optional plus or minus sign then INF or INFINITY, ignoring case.