When a mouse button is pressed inside a window, put that window into an
automatic mouse tracking state where all mouse events are sent to that
window until all mouse buttons are released.
This might feel even better if it only cared about the mouse buttons you
actually pressed while *inside* the windows to get released, I don't know.
I'll have to use it for a while and see how it's like.
- Make it track the mouse cursor just like GButton does so that changes only
get committed if the mouseup event happens while inside the widget rect.
- Draw a focus rect around the box when appropriate.
- When focused, support toggling the checked state with the space bar.
