This PR updates how the "shared" status of a given folder is determined. More specifically, it changes the MagicFolderChecker class/QObject to emit a members_updated signal (replacing member_added, member_removed) which passes a list of currently known members to the folder status Model class. The Model, in turn, updates the UI to display the "shared" emblem/overlay if the list contains more than one member and removes it (thus setting the status to "private") if it contains only one. Previously, the Model tracked membership numbers by way of an integer mapped to each folder name, however this was mistakenly being incremented too often (specifically, when re-scanning a remote folder after it had been removed/deactivated) and was never decremented (thus the status would be displayed as "shared" even if all other members get manually removed from the magic-folder DMD). Passing the full list of currently known members thus helps to ensure that membership information is accurately reflected in the UI and will make it easier, in the future, to perform other membership-related operations from the Model class (like fetching/viewing member "profiles" or removing members from a folder).