* 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.
* MainWindow is ideal for apps that just want a UI and
don't manage their own SDL windows.
* The example app will grow into a series of demos that
test the UI toolkit to help fix bugs and grow features.
