This PR updates how the information represented in the "Status" column is determined and improves upon some adjacent functionality related to how Gridsync parses/handles JSON messages from the Magic-Folder status API. These changes include:

• Adding a "Waiting for scan" status level to represent in the UI the state/time between a) when a folder has finished "Loading" but b) before Magic-Folder has finished determining whether any uploads/downloads are required, fixing "Status" column may remain blank for extended periods of time #461.
• Leveraging the newly added last-scan/last-poll in order to determine more quickly when a folder can be considered "up to date". (Previously, Gridsync had to wait until upload/download operations were observed and completed before making such a determination reliably, as there was no way to tell whether folders weren't syncing because they were already up-to-date or because Magic-Folder had not yet completed a scan/poll).
• Reducing the rate/frequency at which Gridsync calculates a folder's total size and mtime.
• Refactoring, deduplicating, and adding type annotations for modules related to the above (in particular gridsync.gui.model, gridsync.gui.status, and gridsync.gui.systray).