no longer access source db obj in snippet

[sssoleileraaa]

Description

Towards #906 and minimizing access of database objects in the GUI.

Test Plan

1. Regression test the preview snippet
2. Review code to see that we no longer need to worry about source db object deletions in set_snippet because it no longer accesses self.source.collection or self.source.uuid

(Next step/PR is to make sure update doesn't raise an exception in the SourceWidget constructor.)

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

[rmol]

👍 Looks good, I couldn't induce a crash by deleting a source as a message was incoming. Could be down to my rusty gaming reflexes, though.

Snippets for replies, messages, and files were updated correctly. Long snippets were truncated properly.

The only thing I saw was that the return type of get_source_widget should be Optional[QListWidget].