Sliders feature

[johnekj]

This adds sliders feature:

• slider widget using GUI or code: {{widget type="Creatuity\OptimumImages\Block\Widget\Slider" slider_key="home"}}
• slider management in Admin
• assigning images to sliders

[krzysztof-pich]

@johnekj Is it desired for sliders to be part of image optimizer? In my opinion separate module for sliders with require in composer for creatuity/magento2-optimumimages will be better. This way image optimizer will be responsible for optimizing images and slider module will be responsible for sliders instead of one module to rule all image related things :)