Add Preview window to Background set images

[oshelton]

Adding preview dialogs or windows to background set images would be a neat feature.

This would involve the following:

• Change the cursor when hovering over a background set image to a hand.
• When clicked a custom Mahapps.Metro dialog or if that proves problematic investigate using the Mahapps.Metro.SimpleChildWindow rpoject.
• These dialogs should dismiss when clicked anywhere (a small label should be displayed on a semi transparent background somewhere indicating that clicking will close the preview).
• The dialog should fill almost the full size of the window with only a lgiht enough margin to tell that the background has been darkened.