LibGUI: Add a spinbox widget.

This is essentially a combo widget containing a single-line GTextEditor
and two buttons for increment and decrement. The GTextEditor::on_change
callback is hooked to prevent non-numeric input but it's not entirely
perfect since that callback is asynchronous. This will work until we have
some more sophisticated input validation mechanism though.

LibGUI/GSpinBox.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <LibGUI/GWidget.h>
+
+class GButton;
+class GTextEditor;
+
+class GSpinBox : public GWidget {
+public:
+    GSpinBox(GWidget* parent = nullptr);
+    virtual ~GSpinBox() override;
+
+    int value() const { return m_value; }
+    void set_value(int);
+
+    void set_range(int min, int max);
+
+    Function<void(int value)> on_change;
+
+protected:
+    virtual void resize_event(GResizeEvent&) override;
+
+private:
+    GTextEditor* m_editor { nullptr };
+    GButton* m_increment_button { nullptr };
+    GButton* m_decrement_button { nullptr };
+
+    int m_min { 0 };
+    int m_max { 100 };
+    int m_value { 0 };
+};