... and also make `center_x` and `center_y` set
`width` and `height` to `Length::Fill`, respectively.
This targets the most common use case when centering
things and removes a bunch of boilerplate as a result.
* Add optional default behavior
* Add a `default` field
* Add a `default()` method to set the `default` field
* A double-click, ctrl-click or command-click will set the slider to the default value
* Add optional fine-grained control
* Add an optional `step_fine` field
* Add a `step_fine()` method to set the `step_fine` field
* Use `step_fine` in place of `step` while shift is pressed
* Add increment/decrement via up/down keys
* Update `Slider` and `VerticalSlider` examples
