Default thumbnail for audio and video resources that don't have a thumbnail

[glenrobson]

Should we have a default thumbnail for audio and video resources that don't provide a thumbnail e.g:

• No thumb audio only: Simplest Manifest - Audio - ThumbnailPanel
• No thumb video only: Simplest Manifest - Video - ThumbnailPanel

Should we have a fallback image for manifests where we struggle to get a thumbnail e.g:

• Multiple choice of images in a single view - no thumbnail from choice ThumbnailPanel
• Image Rotation Two Ways - image service selector - ThumbnailPanel
• Image Rotation Two Ways - css rotation - ThumbnailPanel

[glenrobson]

Ref #4

[glenrobson]

Probably not important enough to get done by the conference

Pass in a default image but could be empty.