New template varibale

[AlexT]

Add new template variable to change compose page columns width calculations, it may have two values, 'percentages' and 'pixels'.

If it is set to 'pixels' then columns width is calculated in pixels, like now in D 7.0

If it is set to 'percentages' then columns width is calculated in percentages, like in old D 6.1

add it to BxBaseConfig? file, but make it possible to override it in exact template. It is needed for super custom templates which can not use width in pixels to make super custom border.