Added apis to GtkCellArea for GtkIconView purposes.

Added apis to GtkCellArea for GtkIconView purposes.

Added a few apis,

  - GtkCellAreaContext get_preferred_height_for_width &
    width for height apis and vfuncs, this lets the icon view
    request the collective (and aligned) height for width for
    a said row.

  - gtk_cell_area_copy_context() this creates a duplicate of
    an already created and requested context, this way the icon
    view uses a global context to request the widths of all rows
    and then makes a copy with all the stored alignments and
    uses a separate copy to calculate the height and alignments
    of each row separately.