Added extend-select for edit mesh. Note for operator design; you cannot give operators hardcoded properties like "SHIFT" or "ALT", that conflicts with the idea of keymaps. Instead the properties have to be functional, like setting 'extend' on or off. I also think mouse select could get these operators: - activate/select, option 'extend' - activate/select on object center, option 'extend' - activate/select, with menu, option 'extend' and of course loop-select, group-select, and so on. BTW: border-select could also get 'extend' option :)