dicomrectdb - make "marked as done" a separate field in the table

[howff]

At the moment an entry in the DicomTags table means that an image has been examined.
If the user has marked/tagged it then the mark field will be True, and if the mark field is False then the image is "done".
There's no way to say that it's "done" and also that it's "tagged".
This might be a meaningless concept (if it's tagged then it's clearly not "done").
But the GUI is a bit confusing because at the moment you can "tag" an image (which creates an entry and sets the mark) but then when you want to move to the next image and you press N the tag gets removed. (Had you pressed Esc the tag would have been kept, but that means you have to know to press Esc on tagged images and N on untagged images).