Stop selecting a source on sync

[rmol]

Description

Replace SourceList.get_current_source with SourceList.get_selected_source. QListWidget.currentItem() returns something even if no item has been selected, which is not what we want when deciding whether to populate the conversation view after sync.

Fixes #962.

Test Plan

• Start the client. Don't touch nothin'.
• Wait through a sync.
• No source should be selected after the sync; the empty conversation view with instructions should still be shown.

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

[kushaldas]

Works as excepted, approved 👍 🦄