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 :)
0371e6cb21