Panel visual improvements

[emilk]

Before

After

[trivigy]

@emilk This commit creates a regression bug. Because of this (https://github.com/emilk/egui/pull/2261/files#diff-1b1b7f67a664568e1201e4336ca926fa687c506ffaaf2f237a21a1fa4619fab7L715) change, TopBottomPanel forces every usecase to always have a seperator hline irrelevant with regards to .resizable(false) or .frame(Frame { stroke: Stroke::none(), ..Frame::none() }). Those are simply ignored. Prior to this change those were configurable. Should this may be made configurable and still remaining inline with this change to improve the double (stroke + hline) issue?!

Current workaround:

let default_style = ctx.style();
let mut temporary_style = ctx.style().deref().clone();
temporary_style.visuals.widgets.noninteractive.bg_stroke = Stroke::none();
ctx.set_style(temporary_style);
egui::TopBottomPanel::top("top_inset_space")
  .frame(Frame {
    fill: ctx.style().visuals.window_fill(),
    shadow: Shadow::small_dark(),
    ..Frame::none()
  })
  .min_height(insets.top)
  .resizable(false)
  .show(&ctx, |_| {});
ctx.set_style(default_style);

[emilk]

@trivigy good catch. Fixed in df01db2 - you can now set panel.show_separator_line(false) to get the old behavior back