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/Tool.h

@@ -0,0 +1,18 @@
+#pragma once
+
+class GMouseEvent;
+class PaintableWidget;
+
+class Tool {
+public:
+    virtual ~Tool();
+
+    virtual const char* class_name() const = 0;
+
+    virtual void on_mousedown(PaintableWidget&, GMouseEvent&) { }
+    virtual void on_mousemove(PaintableWidget&, GMouseEvent&) { }
+    virtual void on_mouseup(PaintableWidget&, GMouseEvent&) { }
+
+protected:
+    Tool();
+};