Move highlight phrases to new settings system

Add a setting serialize/deserializer for QString
Add constructor to ChatterinoSetting that doesn't take a default value

src/application.cpp

@@ -34,8 +34,6 @@ Application::Application()
 Application::~Application()
 {
     this->save();
-
-    chatterino::singletons::SettingManager::getInstance().save();
 }
 
 int Application::run(QApplication &qtApp)