Tui-rs widget border styling

My PR to introduce double borders and curved corners for styling widget blocks in tui-rs was just merged into master. Not sure when a new release is planned though.

Example usage:

Block::default()
                    .title("With styled and double borders")
                    .border_style(Style::default().fg(Color::Cyan))
                    .borders(Borders::LEFT | Borders::RIGHT)
                    .border_type(BorderType::Double)

and for rounded corners:

Block::default()
                    .title("With rounded corners")
                    .border_style(Style::default().fg(Color::Cyan))
                    .borders(Borders::LEFT | Borders::RIGHT)
                    .border_type(BorderType::Rounded)

Because there are no unicode characters for double arcs, the doubled border and rounded corners are mutually exclusive.