Skin "thumbnails"

[Zemanzo]

Description
Skins currently use their diffuse image as "thumbnail" image in the list in the sidebar. It gives a decent impression, but sometimes lacks in representing the skin. Shader based skins do not have a diffuse image, and thus show up as pure white. The current method could be a decent fallback, but it would be better to have a dedicated image for this.

Minimum Acceptance Criteria

• Skins can have a dedicated thumbnail image that will show up in the list in the sidebar.
• The current method will be used as a fallback, if there is no thumbnail image present.
• If there is no diffuse image as a fallback either, display an error icon instead.