Calculator: Add a simple calculator app

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

Applications/Calculator/CalculatorWidget.h

@@ -0,0 +1,30 @@
+#pragma once
+
+#include "Calculator.h"
+#include "Keypad.h"
+#include <AK/Vector.h>
+#include <LibGUI/GWidget.h>
+
+class GTextBox;
+class GButton;
+class GLabel;
+
+class CalculatorWidget final : public GWidget {
+    C_OBJECT(CalculatorWidget)
+public:
+    explicit CalculatorWidget(GWidget*);
+    virtual ~CalculatorWidget();
+
+private:
+    void add_button(GButton&, Calculator::Operation);
+    void add_button(GButton&, int);
+    void add_button(GButton&);
+
+    void update_display();
+
+    Calculator m_calculator;
+    Keypad m_keypad;
+
+    GTextBox* m_entry { nullptr };
+    GLabel* m_label { nullptr };
+};