Add Thread Guard for debugging simple threading issues (#4254)

* Add ThreadGuard class

* Use ThreadGuard when accessing a ChannelView's messageSnapshot

src/util/ThreadGuard.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <cassert>
+#include <mutex>
+#include <optional>
+#include <thread>
+
+namespace chatterino {
+
+// Debug-class which asserts if guard of the same object has been called from different threads
+struct ThreadGuard {
+#ifndef NDEBUG
+    std::mutex mutex;
+    std::optional<std::thread::id> threadID;
+#endif
+
+    inline void guard()
+    {
+#ifndef NDEBUG
+        std::unique_lock lock(this->mutex);
+
+        auto currentThreadID = std::this_thread::get_id();
+
+        if (!this->threadID.has_value())
+        {
+            this->threadID = currentThreadID;
+            return;
+        }
+
+        assert(this->threadID == currentThreadID);
+#endif
+    }
+};
+
+}  // namespace chatterino