Add a hover style selector

[Zoxc]

This adds a hover style selector to the Style type. It can be used like:

style(|s| s.hover(|s| s.scroll_bar_color(Color::ROYAL_BLUE)));

Currently it only works with the properties defined by the prop! macro.

[jrmoulton]

This is the API I want for all hover/modifier styles. This would make it really easy to make a single style that can be applied to a view that would be fully responsive. Responsiveness using some predefined colors/styles would become beautifully trivial.