Floating scroll bars

[emilk]

• Closes Support floating scrollbars #3316

This introduces settings to control the visuals of scroll bars, with three pre-sets to chose from:

• Solid (old default)
• Thin
• Floating (new default)

To change to the default, use:

ctx.style_mut(|style| {
    style.spacing.scroll = egui::style::ScrollStyle::solid();
});