AttachmentManager tintcolor is readonly, provide a setter for it

[naterock101]

in order to customize the attachment manger we need to make the tintColor property assignable. Right now the backgorund color for the delete button will always be blue.

if we want to change it such as:

let manager = AttachmentManager()
manager.tintColor = UIColor.green

we get an error: Cannot assign to property: 'tintColor' is a get-only property"

a solution could look like:

open var tintColor: UIColor {
    get {
        if #available(iOS 13, *) {
            return .link
        } else {
            return .systemBlue
        }
    }
    set {
        // Implementation to set the value of tintColor
    }
}