Add a render method for the Widget trait that takes by reference instead

[SergeyKasmy]

Description

This PR adds Terminal::render_widget_ref() and (Stateful)Widget::render_ref() functions that take Widgets by reference instead of taking ownership. This allows to call render on a Trait object with unknown size with is useful where the exact object implementing Widget is unknown or depends on some kind of condition. I decided not to change the render_widget() and render() signatures as to keep backwards compatibility but sadly all custom widgets are still broken because they are now required to implement render_ref() instead. This is just a proof of concept PR and I would appreciate any help on getting it up to the standards since this is my first time contributing to a big rust project. I made this to fix issue #526 I posted yesterday and I'm already using this PR in a project of mine.

Checklist

• I have read the contributing guidelines.
• I have added relevant tests.
• I have documented all new additions.

[harrypunk]

Super helpful feature. I'm looking for a way of rendering a dynamic widget.

[kroonhorstdino]

Would definitely like to see this feature merged! Would make my life so much easier, right now you have to work around so much limitations in that regard.