Allow adding post thumb via url

[pascal.herbert]

currently it is only possible to add post thumbs from blog's gallery

This will not work because post_thumbnails are acutally using images on the server, not remote images. I suggest to close as invalid.

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To test this, even if your theme does not uses post thumbnails you can switch them on with:

if ( !isset($_wp_theme_features['post-thumbnails']) )
	$_wp_theme_features['post-thumbnails'] = true; 

Since this is a feature request, I won't suggest to close as invalid any longer. My fault.