Drawing

In the main class, there are 2 components: MyToolBar and CanvasPane.

• MyToolBar has the reference of CanvasPane. When a user selects a shape on the toolbar
  or the selection feature, it'll update the corresponding handler based on the currently
  selected button.
• CanvasPane has a list of shapes to keep track and some methods like clear() and update()
  to refresh the canvas with new shapes.

There are 2 types of event handler, and both of them implement a common interface called
EventResponsible:

• DrawingTool: handles drawing a shape. This class will forward drawing events to a
  specific shape to handle. After the user has done drawing, the shape will notify back to
  this class to add a new shape to the canvas and tell the canvas to update.
• SelectionTool: handles selecting a shape as well as moving and deleting a selected shape.

How to delete a shape?

• Select the Select option on the toolbar.
• Click on the shape to select it.
• Hit Backspace or Delete.

How to draw a polygon?

• Select the Polygon option on the toolbar.
• Continue to click on the canvas to add a vertex to the polygon. The polygon only shows up
  on the screen when there are more than 2 vertices (2 clicks).
• To draw a new polygon after you have done drawing a polygon, press Enter.