Too many saplings from birch leaves

<dotti> milan, air: too many saplings from birch leaves

The default rarity is 20, and the current value of 4 produces 5x too many, and it produces the same number of iska 1&2 saplings, making it 15x as many as default. This patch changes it to 30, adds sticks and reduces the iska saplings to 120, making them 4x as rare as normal saplings. It produces good results for normal birch trees, making the iska drop a little rarer. The iska trees produce more leaves and still drop a lot of saplings, just not as many as they currently do.