New CEGUI grid list functions

[Lord-Henry]

Details
My suggestion is to add 2 new functions related to grid list columns. They are:

1. guiGridListSetColumnNonSortable - This function can be integrated with GuiGridListSetSortingEnabled, adding an optional columnID parameter. If this parameter is not specified (default value is 0), all columns are affected. If specified, the function will only affect the respective column with this ID.

2. guiGridListSetColumnsFixedWidth - This function makes all columns have fixed width, so the player can't change their width using mouse cursor. Would be very useful if there's also an optional columnID parameter.
   (these function names may change)

Additional context
Currently, the useful function guiGridListSetColumnNonSortable may bug if the player change the column width with mouse cursor. Because the "locked click area" has a fixed size and position, based on the column original width.
The inbuilt functions must work in another way to prevent those bugs.