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.

libs/pbd/wscript

@@ -73,6 +73,7 @@ libpbd_sources = [
     'stacktrace.cc',
     'stateful_diff_command.cc',
     'stateful.cc',
+    'string_convert.cc',
     'strreplace.cc',
     'strsplit.cc',
     'system_exec.cc',
@@ -191,6 +192,7 @@ def build(bld):
                 test/mutex_test.cc
                 test/scalar_properties.cc
                 test/signals_test.cc
+                test/string_convert_test.cc
                 test/convert_test.cc
                 test/filesystem_test.cc
                 test/natsort_test.cc