Do not show/use default images

[Nesaijn]

Since I have found the default image to be ugly, I wanted to suggest to remove the use of a default image for the recipes. Especially when many recipes don't have an image it does not look good.

My suggestion would be to not show an image when none is set or the default image is being used. Mainly when opening a recipe but could also apply to the list of the recipes.

[Teifun2]

This is a very good point. I will try to add this to the next release.

[Teifun2]

This is a lot harder than i expected. I am using a library for the Cached Image. And altough i can give different content when the image loading fails. I cannot change the heigth of this content.

I think this could be part of the "caching" overhaul.