Add optional property for defining a custom fullscreen close button.

[thisisaaronland]

This PR tries to build on #385 but rather than defining new presets allows for a consume of the ImageSlideshow package to define a custom UIButton to use for ending fullscreen mode.

The button is assigned to the slideshow's fullscreenCloseButton property which is then assigned to the FullScreenSlideshowViewController's closeButtonCustom property. For example:

        let closeButton = UIButton()
        closeButton.setImage(UIImage(systemName: "xmark"), for: UIControl.State())
        closeButton.tintColor = .white
        
        slideshow.fullscreenCloseButton = closeButton

[dogo]

Replace by if let avoiding the force unwrapping

[thisisaaronland]

Sorry for the slow reply. Just to confirm, this should be:

if let custom_button = closeButtonCustom {
      closeButton = custom_button
}

Or something else?