a11y: Fix refcounting for treeview cells

a11y: Fix refcounting for treeview cells

Old code assumed that AT-SPI would keep track of references and
therefore tried to only hold weak references. On the other hand it also
tried to keep objects alive so it referenced objects very randomly. All
of that lead to cycles and leaking.

As AT-SPI does not keep track of objects at all, the treeview now does.
The refcounting looks as follows:
GtkTreeViewAccessible
  => creates per row/column
GtkTreeViewAccessibleCellInfo
  => which references 1
GtkCellAccessible

If there is only one cell, this accessible is a
GtkRendererCellAccessible, otherwise a GtkContainerCellAccessible is
created and that accessible holds references to the
GtkRendererCellAccessibles that are created for every cell renderer.

https://bugzilla.gnome.org/show_bug.cgi?id=554618