Confusing UX due to Additional Software configuration GUI not refreshing packages list

Steps to reproduce:

1. Start “Additional Software” from the apps menu
2. The list is empty so I follow the instructions, click on the Synaptic “link” in the window to add packages.
3. In Synaptic I install a package
4. I get asked whether I want that package to be installed automatically when I start Tails, and I say yay!
5. I close Synaptic and I’m thus back in the “Additional Software” window which is back on top. But the list in there is still empty: my newly added package is not listed and I wonder if the operation has failed.
6. I verify that the package was correctly added to live-additional-software.conf.

I think step 5 is confusing: after completing the operation (adding a package to my ASP as instructed) I would expect the window that told me to do so to give me up-to-date info. I suspect the same problem exists even if the list is not empty to start with.

Parent Task: #14594 (closed)

Original created by @intrigeri on 15386 (Redmine)