AK: Add a TemporaryChange helper class.

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/781f216676a

AK/TemporaryChange.h

@@ -0,0 +1,18 @@
+#pragma once
+
+namespace AK {
+
+template<typename T>
+class TemporaryChange {
+public:
+    TemporaryChange(T& variable, T value) : m_variable(variable), m_old_value(variable) { m_variable = value; }
+    ~TemporaryChange() { m_variable = m_old_value; }
+
+private:
+    T& m_variable;
+    T m_old_value;
+};
+
+}
+
+using AK::TemporaryChange;