Calculator: Add a simple calculator app

Closes https://github.com/SerenityOS/serenity/issues/319

Applications/Calculator/Calculator.h

@@ -0,0 +1,46 @@
+#pragma once
+
+// This type implements the regular calculator
+// behavior, such as performing arithmetic
+// operations and providing a memory cell.
+// It does not deal with number input; you
+// have to pass in already parsed double
+// values.
+
+class Calculator final {
+public:
+    Calculator();
+    ~Calculator();
+
+    enum class Operation {
+        None,
+        Add,
+        Subtract,
+        Multiply,
+        Divide,
+
+        Sqrt,
+        Inverse,
+        Percent,
+        ToggleSign,
+
+        MemClear,
+        MemRecall,
+        MemSave,
+        MemAdd
+    };
+
+    double begin_operation(Operation, double);
+    double finish_operation(double);
+
+    bool has_error() const { return m_has_error; }
+
+    void clear_operation();
+    void clear_error() { m_has_error = false; }
+
+private:
+    Operation m_operation_in_progress { Operation::None };
+    double m_saved_argument { 0.0 };
+    double m_mem { 0.0 };
+    bool m_has_error { false };
+};