Just have a default 'Misc' category (harmless for panels in non-cat context).
In case we would still want a panel to show in all tabs (rather unlikely),
just explicitely give an empty string to its bl_category property.
Note I choose 'Misc' because it's much shorter than 'Uncategorized' (space
is an issue here), it's a one-liner to change it anyway if UI Mafia does not like it!
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