app: add layers/channels/vectors freeze/thaw to gimpplugin-cleanup
so plug-ins cannot thaw what they haven't frozen, and the code can clean up frozen stuff left behind by crashed or broken plug-ins. Also redo the cleanup code so it only keeps track of the undo group counts and freeze counts of what *this* GimpPlugInProcFrame did itself. That should make it even stricter against broken code that could mess up internals.
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