Move instance fields to a private struct, in preparation
for installing a11y headers.
This also required removing access to GtkRendererCellAccessible innards
from various cell accessible implementations.
1) always emit signals
Previously, newly constructed cells would be told to not emit events.
However, we can ensure that nothing is connected to the signals, so
they will not actually emit anything.
2) don't return anything
The return value is unused anyway.
Various tiny semantic changes in here.
The big fix is that we consider every cell as visible and as showing if
the treeview is mapped.
This was also fixed in the tree dump test.
