Fix operator+= for UTF8 class

Added operator+=(wchar_t), as operator+=(char) was not sufficient to
handle multibyte characters present in UTF8.

Fixes: lp:1737143
* https://bugs.launchpad.net/kicad/+bug/1737143

common/text_utils.cpp

@@ -44,7 +44,10 @@ std::pair<UTF8, std::vector<bool>> ProcessOverbars( const UTF8& aText )
             // If it is a double tilda, just process the second one
         }
 
-        text += *chIt;
+        // remember: *chIt is not necessary a ASCII7 char.
+        // it is an unsigned ( wchar_t ) giving a multibyte char in UTF8 strings
+        text += wchar_t( *chIt );
+
         flags.push_back( overbar );
     }
 

common/utf8.cpp

@@ -218,6 +218,23 @@ UTF8::UTF8( const wchar_t* txt ) :
 }
 
 
+UTF8& UTF8::operator+=( wchar_t ch )
+{
+    if( ch <= 0x7F )
+        m_s.operator+=( char( ch ) );
+    else
+    {
+        wchar_t wide_chr[2];    // buffer to store wide chars (unicode) read from aText
+        wide_chr[1] = 0;
+        wide_chr[0] = ch;
+        UTF8 substr( wide_chr );
+        m_s += substr.m_s;
+    }
+
+    return (UTF8&) *this;
+}
+
+
 #if 0   // some unit tests:
 
 #include <stdio.h>

include/utf8.h

@@ -145,6 +145,10 @@ public:
         return (UTF8&) *this;
     }
 
+    /// Append a wide (unicode) char to the UTF8 string.
+    /// if this char is not a ASCII7 char, it will be added as a UTF8 multibyte seqence
+    UTF8& operator+=( wchar_t ch );
+
     // std::string::npos is not constexpr, so we can't use it in an
     // initializer.
     static constexpr std::string::size_type npos = -1;