Magic-Folder conflicts are not correctly surfaced to the user

[crwood]

Currently, when a conflict within a magic-folder occurs, the folder view's "Status" column will display a status of "Syncing" for that folder -- and will remain in that "Syncing" state indefinitely (seemingly, because, in the event of a conflict, the upload-finished/download-finished event message that Gridsync expects will never be sent by Magic-Folder).

Gridsync should instead do whatever is possible/necessary to a) correctly identify that a conflict has occurred for a given folder and b) present that information to the user in place of the never-ending "Syncing" state (even if, as it currently stands, Magic-Folder provides no API for actually resolving those conflicts; see/follow tahoe-lafs/magic-folder#725 and #76).