... 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