Prevents the adding of items to the ClipboardHistoryModel if the raw
data and mime_type of the item being added is the same as another item
already in the list.
Now that we can copy bitmaps, we need some kind of cap here or memory
usage quickly skyrockets.
This could probably be improved or made adaptive somehow, this is just
a simple hard cap for now.
It will listen for clipboard content changes in the backgroud. Once you click
on its icon, it will pop up a window listing all recorded clipboard contents.
You can then double-click on an item to copy it again.
