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.cpp

@@ -0,0 +1,71 @@
+#include <LibGUI/GSpinBox.h>
+#include <LibGUI/GButton.h>
+#include <LibGUI/GTextEditor.h>
+
+GSpinBox::GSpinBox(GWidget* parent)
+    : GWidget(parent)
+{
+    m_editor = new GTextEditor(GTextEditor::Type::SingleLine, this);
+    m_editor->on_change = [this] {
+        bool ok;
+        int value = m_editor->text().to_uint(ok);
+        if (ok)
+            set_value(value);
+        else
+            m_editor->set_text(String::format("%d", m_value));
+    };
+    m_increment_button = new GButton(this);
+    m_increment_button->set_caption("+");
+    m_increment_button->on_click = [this] (GButton&) { set_value(m_value + 1); };
+    m_decrement_button = new GButton(this);
+    m_decrement_button->set_caption("-");
+    m_decrement_button->on_click = [this] (GButton&) { set_value(m_value - 1); };
+}
+
+GSpinBox::~GSpinBox()
+{
+}
+
+void GSpinBox::set_value(int value)
+{
+    if (value < m_min)
+        value = m_min;
+    if (value > m_max)
+        value = m_max;
+    if (m_value == value)
+        return;
+    m_value = value;
+    m_editor->set_text(String::format("%d", value));
+    update();
+    if (on_change)
+        on_change(value);
+}
+
+void GSpinBox::set_range(int min, int max)
+{
+    ASSERT(min <= max);
+    if (m_min == min && m_max == max)
+        return;
+
+    m_min = min;
+    m_max = max;
+
+    int old_value = m_value;
+    if (m_value < m_min)
+        m_value = m_min;
+    if (m_value > m_max)
+        m_value = m_max;
+    if (on_change && m_value != old_value)
+        on_change(m_value);
+
+    update();
+}
+
+void GSpinBox::resize_event(GResizeEvent& event)
+{
+    int button_height = event.size().height() / 2;
+    int button_width = 16;
+    m_increment_button->set_relative_rect(width() - button_width, 0, button_width, button_height);
+    m_decrement_button->set_relative_rect(width() - button_width, button_height, button_width, button_height);
+    m_editor->set_relative_rect(0, 0, width() - button_width, height());
+}