PaintBrush: Implement a thickness setting for the pen tool.

Painter gains the ability to draw lines with arbitrary thickness.
It's basically implemented by drawing filled rects for thickness>1.

In PaintBrush, Tool classes can now override on_contextmenu() to
provide a context menu for the toolbox button. :^)

Applications/PaintBrush/ToolboxWidget.cpp

@@ -20,6 +20,11 @@ public:
     const Tool& tool() const { return *m_tool; }
     Tool& tool() { return *m_tool; }
 
+    virtual void context_menu_event(GContextMenuEvent& event) override
+    {
+        m_tool->on_contextmenu(event);
+    }
+
 private:
     OwnPtr<Tool> m_tool;
 };