UI: Grid Widget - Enable/Disable Row Selection

Currently, grid widget will style hovered and selected rows and fire
a row selected event when user clicks a row. Though it's confused in
some cases. For example, in software update grid, we only allow user
to "Update All" software packages: user can't select one package and
only update it. So nothing will happen if user clicks one row.

    In this patch, we added the feature that allows grid consumer to
determine disable or enable row selection.

Signed-off-by: Hongliang Wang <hlwang@linux.vnet.ibm.com>