Add a placeholder image property to icon (in the style of fallback)

The name of an icon from the icon theme to show while the icon set in source is being loaded. This is primarily relevant for remote sources, or those using slow- loading image providers. The default temporary icon is "image-x-icon"

This will only be loaded if the source is a type which can be so long-loading that a temporary image makes sense (e.g. a remote image, or from an ImageProvider of the type QQmlImageProviderBase::ImageResponse)

The intention here is to ensure we aren't hardcoding things which would also arguably be useful for the user (specifically, in cases where the user of the component knows more precisely which type of icon is going to be shown here, and consequently have a better choice for the temporary image than a generic icon image).