Make ARIA gridcell cells caret-navigable if browse mode is sticky
Make ARIA gridcell cells caret-navigable if browse mode is sticky

ARIA grids are expected to have author-provided navigation, just like
ARIA's application role. When they don't, Orca's browse mode makes
navigation via the arrows work in the manner expected to have been
provided by the author. But that doesn't provide for caret navigation
within a cell. We solved this problem for badly-authored application-
role use by allowing sticky browse mode to let the user completely take
over navigation. So do the same thing for grid cells.

GNOME/orca,GNOME/orca,GNOME/orca,GNOME/orca