Methods made pub as they may be useful in certain scenarios

Added StyleProp::select helper method

This also seems to have fixed a bug that caused the textbox to jump when
focused (?)

Fixed crashes when no context existed.

- Text refusing to size correctly
- Fonts not loading correctly with new bevy context.

Added border color to styles.

[RFC 1.1] Widget Restructuring

`OnEvent` now takes `KayakContextRef`

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.