Adds placeholderImage property to `Row` protocol

[simonmitchell]

Description

This solves an issue I was having with didSet not being called on a struct/class/enum implementing the Row protocol.

If you want the image view to have a placeholder image, previously the easiest way was to return this placeholder from the image property. However, ThunderTable does a check for nil on the row's image before setting image after an asynchronous image load and therefore didSet on image was never called!

Motivation and Context

Solves problems around loading images for rows

How Has This Been Tested?

Has been tested in the example project

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.