Add option to disable adding images to clipboard history

It is quite slick to have images added to the clipboard history and being able to access such again distinguishing content with a small preview of it. This is especially helpful when accessing the clipboard history through the unity scope where there is a bigger preview available.

However, this has its cost with a higher memory foot print. And furthermore when using photo editing apps like Gimp which is copying frequently this could overflow diodon whit lots of images increasing CPU usage unnecessarily and in most cases those images copied to clipboard won't really be necessary to be added to the clipboard history.

Therefore should be there an option to disable adding of images. Also the impact it can have on memory consumption should it be disabled per default.