Fixes add_row on EvTable class, allowing the creation of a table without a header.
Reworked EvTable to use internal EvColumn objects. This allows a new  method on EvTable. Columns with a set width will not be affected by auto-balancing, allowing for mixing fixed and auto-balancing columns as suggested in #550. Changed EvTable keywords involving characters to consistently end with _char, such as corner_top_left -> corner_top_left_char.
Removed ability to add EvColumns directly to table - this causes some confusing effects and is not worth the trouble over building the table normally and reformatting with reformat_column.