Complete and utter kluge in pursuit of per-display subtabs in layout/layout...

Complete and utter kluge in pursuit of per-display subtabs in layout/layout settings/content editors. Finally gave up and conceded to using access callbacks to control tab visibility.

This commit will be responsibel for the death of at least two dozen kittens. Oh, the things we do in search of D6 local tasks that behave like they did in D5.