Allow inserting LuaGuiElement to specified position

[tux_mark_5]

Right now LuaGuiElement.add inserts newly created child elements at the end of the parent.
It would be nice if there was a way to control where newly created elements are inserted.

This would be especially useful when implementing tables, whose contents can change dynamically.
Right now it seems that you have to clear and recreate the entire table contents just to insert a new row somewhere in the middle.