Implement basic view modifiers

[stackotter]

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:

• padding
• 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.