Add fix and tests for issue #442

[ntoll]

Description

Fixes #442

Adds a new signal, slot and unit tests to ensure the timestamp is updated when new files or messages from the source arrive in the client.

Test Plan

Updated unit tests.

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, packaging logic (e.g., the AppArmor profile) may need to be updated. Please check as applicable:

• I have submitted a separate PR to the packaging repo
• No update to the packaging logic (e.g., AppArmor profile) is required for these changes
• I don't know and would appreciate guidance

[sssoleileraaa]

I like your solution! This works and just needs a rebase.

Edit: Nvm on that rebase, I haven't seen that github feature before where it blocks because a branch is out of date even though it can do a fast-forward rebase.... Oooh this is because @redshiftzero just turned on that github feature