fix crops that drop more than one item

[huancz]

... and few other issues

• harvesting farming crops gave forage XP instead of farming
• it gave constant XP instead of the formula used by the game
• harvesting sunflowers only gave the sunflower itself, not the seeds

I tested it in my greenhouse - harvesting coffe now gives me same amount and same distribution of quality whether I let the grabber do it or do it by hand.