accessibility: Mutex cleanup in AccessibleGridControl* classes

For unknown reasons this uses both SolarMutex and a OBaseMutex::m_aMutex
to lock some but not all methods, which seems odd.

- remove a third mutex AccessibleGridControl::m_aMutex that appears
  unused
- lock OBaseMutex::m_aMutex in AccessibleGridControlBase::isAlive()
  which accesses rBHelper members, which should be sufficient
- lock SolarMutex in all UNO methods
- remove misc weird stuff like TC_SolarMethodGuard

Change-Id: I8321757fb4a5043a05c23f2dc9801f5d8c8d911e
accessibility: don't crash if the TableControl is dead

AccessibleGridControlAccess: clear the m_pTable member in dispose() so
it is no longer accessed later.

Change-Id: I490c84ce1bee55c9c69eb29b81ddfbe808301797
svtools: rename IAccessibleTableControl::dispose()

... to not confuse it with XComponent::dispose().

Change-Id: I056fdf77f9116d1c22f8ca9313a0ea313fb1c4aa