API: 为 Dialog:canvas() 组件添加鼠标进入/退出事件

Describe the solution you'd like In my case, an event that triggers when the mouse exits the canvas could reset the saved mouse position to something that will be outside of any of the custom UI elements. Add an event on the `Dialog:canvas()` component called something like `onmouseexit` or `onmouseout`. To keep it consistent, also add a `onmouseenter` or `onmousein` event, to be set/reset something as soon as the mouse *enters* the canvas.