Dropped temporary images appear in the open recent menu

[DamirPorobic]

On another (but related) issue: The new tabs created by dragging have random names. Even if I close these without saving them, they appear in the "open recent" menu, although they reside in /tmp. I think it would be better to treat these like new captures (as unsaved files without a save location), and ask whether these should be saved when closing. Now there is no question whether one wants to save these, but the files will anyway be deleted from the /tmp directory and not persist there. Plus they litter the "open recent" menu entries.

Steps to reproduce:

1. Create screenshot in tab
2. Drag screenshot into another tab

Originally posted by @pascal-niklaus in #610 (comment)

[DamirPorobic]

Fixed.