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/demo_app_windows.rs

@@ -107,12 +107,12 @@ impl Default for Tests {
     fn default() -> Self {
         Self::from_demos(vec![
             Box::<super::tests::CursorTest>::default(),
+            Box::<super::tests::GridTest>::default(),
             Box::<super::tests::IdTest>::default(),
             Box::<super::tests::InputEventHistory>::default(),
             Box::<super::tests::InputTest>::default(),
             Box::<super::tests::LayoutTest>::default(),
             Box::<super::tests::ManualLayoutTest>::default(),
-            Box::<super::tests::TableTest>::default(),
             Box::<super::tests::WindowResizeTest>::default(),
         ])
     }

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

@@ -1,5 +1,5 @@
 #[derive(PartialEq)]
-pub struct TableTest {
+pub struct GridTest {
     num_cols: usize,
     num_rows: usize,
     min_col_width: f32,
@@ -7,7 +7,7 @@ pub struct TableTest {
     text_length: usize,
 }
 
-impl Default for TableTest {
+impl Default for GridTest {
     fn default() -> Self {
         Self {
             num_cols: 4,
@@ -19,9 +19,9 @@ impl Default for TableTest {
     }
 }
 
-impl crate::Demo for TableTest {
+impl crate::Demo for GridTest {
     fn name(&self) -> &'static str {
-        "Table Test"
+        "Grid Test"
     }
 
     fn show(&mut self, ctx: &egui::Context, open: &mut bool) {
@@ -32,7 +32,7 @@ impl crate::Demo for TableTest {
     }
 }
 
-impl crate::View for TableTest {
+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"),

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

@@ -1,17 +1,17 @@
 mod cursor_test;
+mod grid_test;
 mod id_test;
 mod input_event_history;
 mod input_test;
 mod layout_test;
 mod manual_layout_test;
-mod table_test;
 mod window_resize_test;
 
 pub use cursor_test::CursorTest;
+pub use grid_test::GridTest;
 pub use id_test::IdTest;
 pub use input_event_history::InputEventHistory;
 pub use input_test::InputTest;
 pub use layout_test::LayoutTest;
 pub use manual_layout_test::ManualLayoutTest;
-pub use table_test::TableTest;
 pub use window_resize_test::WindowResizeTest;