Use SystemImage for default close button image

[Mark Pospesel]

See how we used an enum (and unit test) in YTags for the default close button image, and do something similar, except:

1. Update package to require latest YCoreUI 1.7.0+
2. add a new Images enum with a single case xmark (see YTags)
3. conform Images enum to SystemImage
4. set renderingMode to .alwaysTemplate
5. test with size scaling. If problematic, set textStyle to nil (no scaling)
6. add static defaultImage: UIImage property that equals Images.xmark.image (see YTags)
7. use .defaultImage as the default parameter for the close button instead of UIImage(systemName: "xmark"). Update documentation comments and README accordingly if necessary.