Improve egui_extras::Table layout (#4755)

Mostly a refactor, but some minor fixes to how it works.

Mostly preparing for a few bigger changes.

crates/egui_demo_lib/src/demo/tests/grid_test.rs

@@ -0,0 +1,124 @@
+#[derive(PartialEq)]
+pub struct GridTest {
+    num_cols: usize,
+    num_rows: usize,
+    min_col_width: f32,
+    max_col_width: f32,
+    text_length: usize,
+}
+
+impl Default for GridTest {
+    fn default() -> Self {
+        Self {
+            num_cols: 4,
+            num_rows: 4,
+            min_col_width: 10.0,
+            max_col_width: 200.0,
+            text_length: 10,
+        }
+    }
+}
+
+impl crate::Demo for GridTest {
+    fn name(&self) -> &'static str {
+        "Grid Test"
+    }
+
+    fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
+        egui::Window::new(self.name()).open(open).show(ctx, |ui| {
+            use crate::View as _;
+            self.ui(ui);
+        });
+    }
+}
+
+impl crate::View for GridTest {
+    fn ui(&mut self, ui: &mut egui::Ui) {
+        ui.add(
+            egui::Slider::new(&mut self.min_col_width, 0.0..=400.0).text("Minimum column width"),
+        );
+        ui.add(
+            egui::Slider::new(&mut self.max_col_width, 0.0..=400.0).text("Maximum column width"),
+        );
+        ui.add(egui::Slider::new(&mut self.num_cols, 0..=5).text("Columns"));
+        ui.add(egui::Slider::new(&mut self.num_rows, 0..=20).text("Rows"));
+
+        ui.separator();
+
+        let words = [
+            "random", "words", "in", "a", "random", "order", "that", "just", "keeps", "going",
+            "with", "some", "more",
+        ];
+
+        egui::Grid::new("my_grid")
+            .striped(true)
+            .min_col_width(self.min_col_width)
+            .max_col_width(self.max_col_width)
+            .show(ui, |ui| {
+                for row in 0..self.num_rows {
+                    for col in 0..self.num_cols {
+                        if col == 0 {
+                            ui.label(format!("row {row}"));
+                        } else {
+                            let word_idx = row * 3 + col * 5;
+                            let word_count = (row * 5 + col * 75) % 13;
+                            let mut string = String::new();
+                            for word in words.iter().cycle().skip(word_idx).take(word_count) {
+                                string += word;
+                                string += " ";
+                            }
+                            ui.label(string);
+                        }
+                    }
+                    ui.end_row();
+                }
+            });
+
+        ui.separator();
+        ui.add(egui::Slider::new(&mut self.text_length, 1..=40).text("Text length"));
+        egui::Grid::new("parent grid").striped(true).show(ui, |ui| {
+            ui.vertical(|ui| {
+                ui.label("Vertical nest1");
+                ui.label("Vertical nest2");
+            });
+            ui.label("First row, second column");
+            ui.end_row();
+
+            ui.horizontal(|ui| {
+                ui.label("Horizontal nest1");
+                ui.label("Horizontal nest2");
+            });
+            ui.label("Second row, second column");
+            ui.end_row();
+
+            ui.scope(|ui| {
+                ui.label("Scope nest 1");
+                ui.label("Scope nest 2");
+            });
+            ui.label("Third row, second column");
+            ui.end_row();
+
+            egui::Grid::new("nested grid").show(ui, |ui| {
+                ui.label("Grid nest11");
+                ui.label("Grid nest12");
+                ui.end_row();
+                ui.label("Grid nest21");
+                ui.label("Grid nest22");
+                ui.end_row();
+            });
+            ui.label("Fourth row, second column");
+            ui.end_row();
+
+            let mut dyn_text = String::from("O");
+            dyn_text.extend(std::iter::repeat('h').take(self.text_length));
+            ui.label(dyn_text);
+            ui.label("Fifth row, second column");
+            ui.end_row();
+        });
+
+        ui.vertical_centered(|ui| {
+            egui::reset_button(ui, self, "Reset");
+            ui.add(crate::egui_github_link_file!());
+        });
+    }
+}