Implemented "Copy to" for the thumbnails view

With this patchset, copying the currently selected image or all selected images from the thumbnail view is possible by a context menu action, like it is now from the viewer for the currently viewed image.

I'm not perfectly sure if it's meaningful to implement this in it's own class; I simply took the ExternalPopup class as a template.

Anyways, I think this is a feature as nice and handy as the "Copy to" in the viewer, and it makes it possible to copy a whole search result.