Implement basic view modifiers

[stackotter]

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SwiftUI has a concept called view modifiers. SwiftUI's ViewModifier protocol is for modifiers that can apply to any view, but individual views can also declare their own modifiers by creating a method of the form func modifierName(/* parameters */) -> Self which modifies the view accordingly and returns a copy.

The following view modifiers would be the most useful to have:

• [x] padding
• [x] foreground (colour)
• [ ] background (colour)
• [ ] scale

If you think of any more, reply below.

~~The SwiftGtk package currently doesn't support CSS styling which means that the styling feature request for SwiftGtk would have to be completed first.~~ SwiftGtk now supports CSS styling and has been merged into the SwiftCrossUI repo for ease of rapid development.