LibGUI: Start working on a GScrollBar.

This widget is far from finished, but it's off to a good start.
Also added a GResizeEvent and GWidget::resize_event() so that widgets
can react to being resized.

LibGUI/GScrollBar.cpp

@@ -0,0 +1,120 @@
+#include <LibGUI/GScrollBar.h>
+#include <SharedGraphics/GraphicsBitmap.h>
+#include <SharedGraphics/Painter.h>
+
+GScrollBar::GScrollBar(GWidget* parent)
+    : GWidget(parent)
+{
+}
+
+GScrollBar::~GScrollBar()
+{
+}
+
+void GScrollBar::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 GScrollBar::set_value(int value)
+{
+    if (value < m_min)
+        value = m_min;
+    if (value > m_max)
+        value = m_max;
+    if (value == m_value)
+        return;
+    m_value = value;
+    if (on_change)
+        on_change(value);
+    update();
+}
+
+Rect GScrollBar::up_button_rect() const
+{
+    return { 0, 0, button_size(), button_size() };
+}
+
+Rect GScrollBar::down_button_rect() const
+{
+    return { 0, height() - button_size(), button_size(), button_size() };
+}
+
+Rect GScrollBar::pgup_rect() const
+{
+    return { 0, button_size(), button_size(), scrubber_rect().top() - button_size() };
+}
+
+Rect GScrollBar::pgdn_rect() const
+{
+    auto scrubber_rect = this->scrubber_rect();
+    return { 0, scrubber_rect.bottom(), button_size(), height() - button_size() - scrubber_rect.bottom() };
+}
+
+Rect GScrollBar::scrubber_rect() const
+{
+    int range_size = m_max - m_min;
+    if (range_size == 0)
+        return { 0, button_size(), button_size(), height() - button_size() * 2 };
+    float available_y = height() - button_size() * 3;
+    float y_step = available_y / range_size;
+    float y = button_size() + (y_step * m_value);
+    return { 0, (int)y, button_size(), button_size() };
+}
+
+void GScrollBar::paint_event(GPaintEvent&)
+{
+    Painter painter(*this);
+
+    painter.fill_rect(rect(), Color(0xc0c0c0));
+
+    painter.draw_rect(up_button_rect(), Color::DarkGray, true);
+    painter.fill_rect(up_button_rect().shrunken(2, 2), Color::LightGray);
+
+    painter.draw_rect(down_button_rect(), Color::DarkGray, true);
+    painter.fill_rect(down_button_rect().shrunken(2, 2), Color::LightGray);
+
+    painter.fill_rect(pgup_rect(), Color::Magenta);
+    painter.fill_rect(pgdn_rect(), Color::Yellow);
+
+    painter.fill_rect(scrubber_rect(), Color::Red);
+
+    dbgprintf("Paint!\n");
+}
+
+void GScrollBar::mousedown_event(GMouseEvent& event)
+{
+    if (event.button() != GMouseButton::Left)
+        return;
+    if (up_button_rect().contains(event.position())) {
+        set_value(value() - m_step);
+        return;
+    }
+    if (down_button_rect().contains(event.position())) {
+        set_value(value() + m_step);
+        return;
+    }
+    if (pgup_rect().contains(event.position())) {
+        set_value(value() - m_big_step);
+        return;
+    }
+    if (pgdn_rect().contains(event.position())) {
+        set_value(value() + m_big_step);
+        return;
+    }
+}