A slider UI widget

## What problem does this solve or what need does it fill?

Currently there are three premade UI elements available: buttons, images and text. I think a slider widget would be useful, e.g. for settings menus. In the future it could be also used in the editor.

## What solution would you like?

A `SliderBundle`, similar to `ButtonBundle` or `TextBundle`, but with a `Slider` component containing slider specific values, like max and min value. It would also contain a queryable component similar to `Interaction`, holding the current value of the slider.

## What alternative(s) have you considered?

Alternatively we could just leave it for the user or a third-party library to implement, but I think that a slider is such a common UI element that it should be included in Bevy itself.