* Added the TabFrame widget with an example program and screenshot
* Button: FixedColor=true to set a consistent background color and not
worry about it with mouseover/down events.
* Tooltip can be added to any target widget (e.g. Button) and pop up on
mouse over.
* Refactor the event system. Instead of passing a render.Point to all
event handlers, pass an EventData struct which can hold the Point or
the render.Engine.
* Add event types Computed and Present, so a widget can set a handler on
whenever its Computed or Present method is called.
