Serve `Frame` corners as handy getters

API suggestion:

```swift
frame.corner.tl
frame.corner.tr
frame.corner.bl
frame.corner.br

frame.middle // a middle point of frame rectangle
frame.middle.top // a middle point of top edge
frame.middle.bottom
frame.middle.right
frame.middle.left
```

All above getters will return `CGPoint` struct. 🤔 