#121 Adding additional center-bottom column

[karedokx]

improve the current xoops template standard to have an
additional 'center-bottom' column:

http://www.tungning.com/misc/layout2.gif

this addition will bring a whole new range of layouting
capability to xoops. currently most of the xoops sites
(including www.xoops.org) only using the center-center
column layout which i believe due to this limitation.

an additional field is to add in the 'block administration'
screen:

side
left-centertop-centerbottom-right

we should not have a backward compability issue too.