Add wxString ctor from wxScopedCharBuffer and wxMBConv

This is more convenient and less error prone than using the existing
ctor taking char pointer and length as the buffer contains both.

Also add the corresponding assign() overload for consistency.

See #16490.

include/wx/string.h

@@ -1162,6 +1162,9 @@ public:
   wxString(const wxScopedWCharBuffer& buf)
     { assign(buf.data(), buf.length()); }
 
+  wxString(const wxScopedCharBuffer& buf, const wxMBConv& conv)
+    { assign(buf, conv); }
+
     // NB: this version uses m_impl.c_str() to force making a copy of the
     //     string, so that "wxString(str.c_str())" idiom for passing strings
     //     between threads works
@@ -2539,6 +2542,13 @@ public:
     { return assign(str.AsString()); }
   wxString& assign(const wxScopedCharBuffer& str)
     { return assign(str.data(), str.length()); }
+  wxString& assign(const wxScopedCharBuffer& buf, const wxMBConv& conv)
+  {
+      SubstrBufFromMB str(ImplStr(buf.data(), buf.length(), conv));
+      m_impl.assign(str.data, str.len);
+
+      return *this;
+  }
   wxString& assign(const wxScopedWCharBuffer& str)
     { return assign(str.data(), str.length()); }
   wxString& assign(const wxCStrData& str, size_t len)