Instead of storing the cached manager info in the Blender preferences,
store the info in a JSON file. The file is located in the user prefs
folder (`~/.config/blender/{version}/config` on Linux).
This also reduces the number of 'refresh' operators to a single one, which
then fetches all necessary info from the Manager.
This fixes an issue (reported via chat) where worker tags were sometimes
not retained across file saves.
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