QuickShow: Allow panning and zooming the image instead of stretching it.

This needs more work and polish, but it's a step in a more pleasant and
useful direction.

Also turn QuickShow into a fully-fledged "application". (By that, I really
just mean giving it its own Applications/ subdirectory.)

Applications/QuickShow/QSWidget.cpp

@@ -0,0 +1,85 @@
+#include "QSWidget.h"
+#include <LibGUI/GPainter.h>
+#include <SharedGraphics/GraphicsBitmap.h>
+
+QSWidget::QSWidget(GWidget* parent)
+    : GFrame(parent)
+{
+    set_frame_shape(FrameShape::Container);
+    set_frame_shadow(FrameShadow::Sunken);
+    set_frame_thickness(2);
+
+    set_background_color(Color::White);
+}
+
+QSWidget::~QSWidget()
+{
+}
+
+void QSWidget::set_bitmap(NonnullRefPtr<GraphicsBitmap> bitmap)
+{
+    set_fill_with_background_color(bitmap->has_alpha_channel());
+    m_bitmap = move(bitmap);
+}
+
+void QSWidget::relayout()
+{
+    Size new_size;
+    float scale_factor = (float)m_scale / 100.0f;
+    new_size.set_width(m_bitmap->width() * scale_factor);
+    new_size.set_height(m_bitmap->height() * scale_factor);
+    m_bitmap_rect.set_size(new_size);
+    update();
+}
+
+void QSWidget::resize_event(GResizeEvent& event)
+{
+    relayout();
+    GWidget::resize_event(event);
+}
+
+void QSWidget::paint_event(GPaintEvent& event)
+{
+    GPainter painter(*this);
+    painter.add_clip_rect(event.rect());
+
+    painter.draw_scaled_bitmap(m_bitmap_rect, *m_bitmap, m_bitmap->rect());
+}
+
+void QSWidget::mousedown_event(GMouseEvent& event)
+{
+    if (event.button() != GMouseButton::Left)
+        return;
+    m_pan_origin = event.position();
+    m_pan_bitmap_origin = m_bitmap_rect.location();
+}
+
+void QSWidget::mouseup_event(GMouseEvent& event)
+{
+    UNUSED_PARAM(event);
+}
+
+void QSWidget::mousemove_event(GMouseEvent& event)
+{
+    if (!(event.buttons() & GMouseButton::Left))
+        return;
+
+    auto delta = event.position() - m_pan_origin;
+    m_bitmap_rect.set_location(m_pan_bitmap_origin.translated(delta));
+    update();
+}
+
+void QSWidget::mousewheel_event(GMouseEvent& event)
+{
+    auto old_scale = m_scale;
+    m_scale += -event.wheel_delta() * 10;
+    if (m_scale < 10)
+        m_scale = 10;
+    if (m_scale > 1000)
+        m_scale = 1000;
+    relayout();
+    if (old_scale != m_scale) {
+        if (on_scale_change)
+            on_scale_change(m_scale);
+    }
+}