Feature: Thumbnail/previews for different OS's

[sconklin]

It will look better if pattern files (.val) will support preview in explorer.

Useful links:

• Gnome. Installing a Thumbnailer Program.
• Windows. SVG Viewer Extension for Windows Explorer
• Windows. Creating Shell Extension Handlers
• Windows. IThumbnailProvider interface
• Windows. Thumbnail Handlers
• KDE. ThumbCreator Class Referenceabstract