PaintBrush: Implement a naive but working bucket fill tool.

I've used a SinglyLinkedList<Point> for the flood fill queue, since Vector
was death slow. This could definitely be made faster with a better algorithm
and/or data structure. :^)

Applications/PaintBrush/BucketTool.cpp

@@ -1,4 +1,8 @@
 #include "BucketTool.h"
+#include "PaintableWidget.h"
+#include <AK/SinglyLinkedList.h>
+#include <LibGUI/GPainter.h>
+#include <SharedGraphics/GraphicsBitmap.h>
 #include <stdio.h>
 
 BucketTool::BucketTool()
@@ -9,7 +13,35 @@ BucketTool::~BucketTool()
 {
 }
 
-void BucketTool::on_mousedown(PaintableWidget&, GMouseEvent&)
+static void flood_fill(GraphicsBitmap& bitmap, const Point& start_position, Color target_color, Color fill_color)
 {
-    dbgprintf("FIXME: Implement BucketTool::on_mousedown\n");
+    SinglyLinkedList<Point> queue;
+
+    queue.append(Point(start_position));
+    while (!queue.is_empty()) {
+        auto position = queue.take_first();
+        if (!bitmap.rect().contains(position))
+            continue;
+        if (bitmap.get_pixel(position) != target_color)
+            continue;
+        bitmap.set_pixel(position, fill_color);
+
+        queue.append(position.translated(0, -1));
+        queue.append(position.translated(0, 1));
+        queue.append(position.translated(1, 0));
+        queue.append(position.translated(-1, 0));
+    }
+}
+
+void BucketTool::on_mousedown(PaintableWidget& paintable_widget, GMouseEvent& event)
+{
+    if (!paintable_widget.rect().contains(event.position()))
+        return;
+
+    GPainter painter(paintable_widget.bitmap());
+    auto target_color = paintable_widget.bitmap().get_pixel(event.x(), event.y());
+
+    flood_fill(paintable_widget.bitmap(), event.position(), target_color, paintable_widget.color_for(event));
+
+    paintable_widget.update();
 }