Add overline option for Table rows (#5637)

* Closes no issue, I just needed this for an app and figured it could be
useful.
* [x] I have followed the instructions in the PR template

This PR adds an `overline` option for `egui_extras::TableRow`, which is
useful for visually grouping rows. The overline consumes no layout
space.

A screenshot of the demo app, showing every 7th row getting an overline.

<img width="704" alt="Screenshot 2025-01-25 at 14 40 08"
src="https://github.com/user-attachments/assets/9ccbee3d-296d-4afd-9290-c669e4ede1c0"
/>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_extras/src/layout.rs

@@ -33,6 +33,7 @@ pub(crate) struct StripLayoutFlags {
     pub(crate) striped: bool,
     pub(crate) hovered: bool,
     pub(crate) selected: bool,
+    pub(crate) overline: bool,
 
     /// Used when we want to accruately measure the size of this cell.
     pub(crate) sizing_pass: bool,
@@ -232,6 +233,14 @@ impl<'l> StripLayout<'l> {
             child_ui.style_mut().visuals.override_text_color = Some(stroke_color);
         }
 
+        if flags.overline {
+            child_ui.painter().hline(
+                max_rect.x_range(),
+                max_rect.top(),
+                child_ui.visuals().widgets.noninteractive.bg_stroke,
+            );
+        }
+
         add_cell_contents(&mut child_ui);
 
         child_ui