Handle drag-and-drop of WARCs onto dock icon

[machawk1]

I originally anticipated doing this in machawk1#82 but with WAIL-Electron, the dynamics may need to be different. The expected behavior might be list the collections in a high level interface for the user to choose to which collection the WARC should be added. For WAIL-py, this would have been a simple copy procedure (i.e., there was no concept of collection). Dragging onto the macOS dock icon would be a familiar convention to users.