Thanks to `impl From<bool> for Vec2b` one can now shorten some builder
calls, like:
Previous:
```rust
egui::ScrollArea::vertical()
.auto_shrink([false; 2])
```
New:
```rust
egui::ScrollArea::vertical()
.auto_shrink(false)
```
* Add Row::text
* Rename elide_at_width -> truncate_at_width
* Move text layout tests to own module
* Add test to check that elision character is always included
* Include elision character in more circumstances
* Append overflow character if we can't replace
* Always append … when eliding
* Add a secondary text to the text layout demo
* Add `TextFormat::extra_letter_spacing`
* Add control of line height
* Add to text layout demo
* Move the text layout demo to its own window in the demo app
* Fix doclink
* Better document points vs pixels
* Better documentation and code cleanup
