* Added `BoxSize()` to Widget that reports the full box size including
borders and margin.
* The Frame uses the `BoxSize()` of widgets to position them.
Reintroduces some padding issues (boxes on the GUI Test stick out of
bounds a bit) but is on the right track.
* Renamed `Padding` to `Margin` on the Widget object, since the Margin
is taken into consideration along with Outline and Border in computing
the widget's BoxSize.
* Restructured the Label widget to take a Text or TextVariable property
and the font settings (render.Text) are in a new `Font` property.
* 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.
The Buttons can now be managed by a ui.Supervisor and be notified when
the mouse enters or leaves their bounding box and handle click events.
Current event handlers supported:
* MouseOver
* MouseOut
* MouseDown
* MouseUp
* Click
Each of those events are only fired when the state of the event has
changed, i.e. the first time the mouse enters the widget MouseOver is
called and then when the mouse leaves later, MouseOut is called.
A completed click event (mouse was released while pressed and hovering
the button) triggers both MouseOut and Click, so the button can pop
itself out and also run the click handler.
* The shell now supports an "eval" command, or "$" for short.
* Runs it in an Otto JavaScript VM.
* Some global variables are available, like `d` is the Doodle object
itself, `log`, `RGBA()` and `Point()`
* The shell supports paging through input history using the arrow keys.
* Added an initial Main Scene
