Added `WidgetProps`

This allows copy values to be used directly from props in rsx (i.e.
`props.copy_value`)

Added WidgetProps::set_children(...) in order to achieve this

Props no longer need to be named "<Widget Name>Props"

This reverts commit 5bca7d20.

This code doesn't work though. Running it shows that widgets are not
aligned/sized properly anymore. Will need fixing.

Also added trait bounds to Widget since its trait object has been replaced by
BaseWidget.

Swapped us over to a `KayakContextRef` which better handles some things.

Border

[RFC 1] Widget Restructuring

Draggable Windows