Consider introducing a new API type to represent case-insensitive filenames

[Ortham]

LOOT has to perform some case-insensitive string comparisons when working with filenames that may not correspond to files that actually exist, but it's not obvious when a string is treated case-insensitively. Introducing a new wrapper type could help with that.

The wrapper type would look something like:

class Filename {
public:
  explicit Filename();
  explicit Filename(const std::string& filename);

  std::string ToString() const; // Or overload operator<< for ostream, or implement an explicit cast operator.
private:
  std::string filename_;
};

// Also implement comparison operators using CompareFilename and specialise 
// std::hash using NormalizeFilename.

Things that would use this new Filename class:

• File::name_
• PluginMetadata::name_
• PluginInterface::name_
• PluginInterface::GetMasters()
• PluginSortingData::GetMasters()
• PluginSortingData::GetAfterGroupPlugins()
• LoadOrderHandler::GetImplicitlyActivePlugins()

[Ortham]

Done in 15d98c9 and 062ab24.