* ncplane_at_* and ncplane_at_cursor_*
We had notcurses_at_yx() expanding into three distinct parts of
the cell structure, and ncplane_at_yx() / ncplane_at_cursor()
writing directly to a cell. It was annoying to remember which
was which. The latter two now have a signature matching
notcurses_at_yx(), while the old functionality has been moved
to ncplane_at_yx_cell() and ncplane_at_cursor_yx(). #476
