CheckButton is a generic component based on Button that additionally
takes a *bool variable to manage. When the CheckButton is clicked or
unclicked, it will toggle the bool var and its border style will "stick"
in or out depending on the state.
Checkbox is a Frame widget that wraps a CheckButton and another child
widget, such as a Label. Interacting with the child widget will forward
all of its mouse events to the CheckButton, so that the Label could be
clicked instead of just the box itself.
* Frame.Pack() now supports Fill and Expand and works like Tk.
* The GUITest Scene now draws a large window with two fixed side panels,
an expanding body panel, and a fixed footer with buttons. The panels
are filled with other buttons and widgets showing off the Frame
packing.
