Add in new convenience function for a hidden Scrollbar

[ellieplayswow]

why

• a Scrollable widget requires a Direction, which in turn requires a Scrollbar
• all of these are reasonably defaulted to a vertical-only scrollable with a sane 8px wide scrollbar
• sometimes you want to maintain the scrollable functionality but not display a scrollbar

changes tl;dr

• adding in convenience function iced_widget::scrollable::Scrollbar::hidden() that is simply default() with zero-width

example:

widget::scrollable(row![
    horizontal_space().width(Length::Fixed(600.0)),
    "hello",
    horizontal_space().width(Length::Fixed(600.0))
])
.direction(Direction::Horizontal(Scrollbar::hidden()))
// you can then control via methods eg iced::widget::scrollable::scroll_to(id, offset)

[hecrj]

Merged manually, since I wasn't able to push to your branch.