Fix storing UTF-8 configuration values on Windows

std::string values were accidentally converted to wxString using current
charset instead of UTF-8, eventually resulting in double-encoded UTF-8.

Fix by explicitly treating narrow strings as UTF-8 in both directions.

The actual fix is using str::to_wx(value) instead of wxString(value)
in Config::Write(key, std::string); the rest of the changes is just
for consistency.

src/configuration.cpp

@@ -220,14 +220,14 @@ bool Config::Read(const std::string& key, std::string *out)
     wxString s;
     if (!wxConfig::Get()->Read(key, &s))
         return false;
-    *out = s.utf8_string();
+    *out = str::to_utf8(s);
     return true;
 }
 
 void Config::Write(const std::string& key, const std::string& value)
 {
     CfgLock lock;
-    wxConfig::Get()->Write(key, wxString(value));
+    wxConfig::Get()->Write(key, str::to_wx(value));
 }
 
 bool Config::Read(const std::string& key, std::wstring *out)
@@ -237,14 +237,14 @@ bool Config::Read(const std::string& key, std::wstring *out)
     wxString s;
     if (!wxConfig::Get()->Read(key, &s))
         return false;
-    *out = s.ToStdWstring();
+    *out = str::to_wstring(s);
     return true;
 }
 
 void Config::Write(const std::string& key, const std::wstring& value)
 {
     CfgLock lock;
-    wxConfig::Get()->Write(key, wxString(value));
+    wxConfig::Get()->Write(key, str::to_wx(value));
 }
 
 bool Config::Read(const std::string& key, bool *out)

src/str_helpers.h

@@ -115,6 +115,11 @@ inline wxString to_wx(const std::string& utf8)
     return wxString::FromUTF8(utf8.c_str());
 }
 
+inline wxString to_wx(const std::wstring& str)
+{
+    return wxString(str);
+}
+
 #if defined(__cplusplus) && defined(__OBJC__)
 
 inline NSString *to_NS(const wxString& str)