This PR adds some parsing/monitoring routines for tracking individual file Magic-Folder upload/download operations, subsequently enabling progress-indicators for Magic-Folder upload operations (displayed in the form of a text-based percentage (e.g., "Syncing (75%)") in the "Status" column for each folder) and reinstating desktop notifications for newly-updated files. Due to current limitations with both Tahoe-LAFS and the Magic-Folder status API, however, this sync-progress value is presently calculated from the number of operations queued against the number of operations completed only; it does not take into account the number of bytes transferred/remaining (but may in the future, if/when Tahoe-LAFS and Magic-Folder provide such information). Accordingly, this indicator should not be relied upon to, e.g., estimate the amount of time remaining until a given folder has finished synchronizing (but is, at least, better than nothing...).

Note, also, that at the time of writing, the progress indicator only tracks upload operations; progress indicators for download operations are currently broken as a result of tahoe-lafs/magic-folder#589.