Added: Img skins can now be scaled whilst maintaining aspect ratio

If keepAspect is set to true image skins will maintain aspect ratio
whilst scaling (and be automatically centered)
Added: Img skins can now also crop while being scaled to fill the widget space
Merge branch 'master' of https://github.com/ProPuke/StablexUI into ProPuke-master
merge #170 skin `Img.crop`