Implement tabs as spaces

When we load a string in `TextEdit` we keep the tabs as they are.

Now depending on `tab_as_spaces` and `tab_size` properties of the
`TextEdit` we have the following situations:

- When `tab_as_spaces` is false, then when we press `TAB` a `TAB`
  char in added to the string.

- When `tab_as_spaces` is true, then when we press `TAB` a chunck
  of `tab_size` spaces is added to the string.

To config `tab_as_spaces` and `tab_size` to the values we want we
can use the folliwing helper functions:

- `TextEdit::tab_as_spaces()`
- `TextEdit::tab_size()`

Example usage:

```rust
ui.add(egui::TextEdit::multiline(&mut self.multiline_text_input)
    .tab_as_spaces(self.tab_as_spaces)
    .tab_size(self.tab_size));
```

Demo:
A better demo of this new functionality was implemented in the
misc widgets demo

What's left to do:
- Render actual `TAB` as a rect the size of `tab_size` spaces.

- Without default parameters values it seems a pain to implement.

  I need to think if it is possible to avoid API breaking changes
  for the font system. From my checks so far I need to pass at
  least `tab_size` to the fonts system to know how big the glyph
  for `TAB` will be.

egui_demo_lib/src/apps/demo/widgets.rs

@@ -23,6 +23,8 @@ pub struct Widgets {
     angle: f32,
     color: Color32,
     single_line_text_input: String,
+    tab_size: usize,
+    tab_as_spaces: bool,
     multiline_text_input: String,
 }
 
@@ -35,7 +37,16 @@ impl Default for Widgets {
             angle: std::f32::consts::TAU / 3.0,
             color: (Rgba::from_rgb(0.0, 1.0, 0.5) * 0.75).into(),
             single_line_text_input: "Hello World!".to_owned(),
-            multiline_text_input: "Text can both be so wide that it needs a line break, but you can also add manual line break by pressing enter, creating new paragraphs.\nThis is the start of the next paragraph.\n\nClick me to edit me!".to_owned(),
+
+            tab_size: 4,
+            tab_as_spaces: true,
+            multiline_text_input: r#"Text can both be so wide that it needs a line break, but you can also add manual line break by pressing enter, creating new paragraphs.
+This is the start of the next paragraph.
+
+Existing	tabs	are	kept	as	tabs.
+Use the configs above to set how new tabs should be handled and test by pressing Tab.
+
+Click me to edit me!"#.to_owned(),
         }
     }
 }
@@ -133,7 +144,22 @@ impl Widgets {
             }
         });
 
-        ui.label("Multiline text input:");
-        ui.text_edit_multiline(&mut self.multiline_text_input);
+        ui.horizontal(|ui| {
+            ui.label("Multiline text input:");
+
+            ui.separator();
+
+            ui.add(egui::Slider::usize(&mut self.tab_size, 2..=8).text("Tab size"));
+
+            ui.separator();
+
+            ui.checkbox(&mut self.tab_as_spaces, "Tabs as spaces");
+        });
+
+        ui.add(
+            egui::TextEdit::multiline(&mut self.multiline_text_input)
+                .tab_as_spaces(self.tab_as_spaces)
+                .tab_size(self.tab_size),
+        );
     }
 }