Allow removing scans

[jonnius]

I'd say, we should have:

• A "new session" button that simply clears the app (QML: ImageProcessing.removeAll())
• Some way to remove just one image from the session (QML: ImageProcessing.removeImage(model.imageID))

[jonnius]

Remove a single image by holding in.

[cibersheep]

This is a nice place to come back :)