correctly dispose to avoid cyclic dependencies

accessibility cruft is still listening to dead tables so crashes in
slidesorting in main panel if moved slide has tables in it after visiting slide
sorter once.

Change-Id: I09f9a73b01fb2ddf059402146acdc7bd823798b9
disposed but not dtored

just die when you are supposed to, without this endless amounts of accessiblity
cells remain after sorting slides with tables in them

Change-Id: Ice9a86b8b806e58f9bf871341a38f7729798dda9