Add cached emotes fallback for failed provider fetches (#6125)

src/util/Helpers.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <QColor>
+#include <QJsonDocument>
 #include <QLocale>
 #include <QString>
 
@@ -218,4 +219,11 @@ void removeFirstQS(QString &str);
 /// @param str The Qt string we want to remove 1 character from
 void removeLastQS(QString &str);
 
+void writeProviderEmotesCache(const QString &id, const QString &provider,
+                              const QByteArray &bytes);
+
+bool readProviderEmotesCache(
+    const QString &id, const QString &provider,
+    const std::function<void(QJsonDocument)> &callback);
+
 }  // namespace chatterino