Calculator: Add a simple calculator app

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

Applications/Calculator/Keypad.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <AK/AKString.h>
+
+// This type implements number typing and
+// displaying mechanics. It does not perform
+// any arithmetic operations or anything on
+// the values it deals with.
+
+class Keypad final {
+public:
+    Keypad();
+    ~Keypad();
+
+    void type_digit(int digit);
+    void type_decimal_point();
+    void type_backspace();
+
+    double value() const;
+    void set_value(double);
+
+    String to_string() const;
+
+private:
+    // Internal representation ofthe current decimal value.
+    bool m_negative { false };
+    long m_int_value { 0 };
+    long m_frac_value { 0 };
+    int m_frac_length { 0 };
+    // E.g. for -35.004200,
+    // m_negative = true
+    // m_int_value = 35
+    // m_frac_value = 4200
+    // m_frac_length = 6
+
+    enum class State {
+        External,
+        TypingInteger,
+        TypingDecimal
+    };
+
+    State m_state { State::External };
+};