The style we use for callbacks in widgets is "onFoo". These classes were using
an order naming convention and just called their callbacks "callback".

Added horizontal and vertical alignment properties to Stack so that the origin of non-positioned children can be specified. Currently all of the non-positioned children just end up with their top-left at 0,0. Now, for example, you can center the children by specifying verticalAlignment: 0.5, horizontalAlignment: 0.5.

Added IndexedStack which only paints the stack child specified by the index property. Since it's a Stack, it's as big as the biggest non-positioned child. This component will be essential for building mobile drop down menus.

Added a (likely temporary) example that demonstrates IndexedStack.

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

This patch makes ParentDataNode less general purpose and instead teaches Flex
and Stack how to program the parent data for their children. We used to have
this general system because parent data used to carry CSS styling, but we don't
need it anymore.

Fixes #957