PaintBrush: Factor out the pen tool into an actual PenTool class.

Also add a Tool base class, and an empty BucketTool subclass which is the
next thing to implement.

Applications/PaintBrush/BucketTool.cpp

@@ -0,0 +1,15 @@
+#include "BucketTool.h"
+#include <stdio.h>
+
+BucketTool::BucketTool()
+{
+}
+
+BucketTool::~BucketTool()
+{
+}
+
+void BucketTool::on_mousedown(PaintableWidget&, GMouseEvent&)
+{
+    dbgprintf("FIXME: Implement BucketTool::on_mousedown\n");
+}