Set the polycount with the decimation modifier

The Decimation modifier doesn't allow to set the polycount but only to set a ratio between 0 and 1. I suggest to be able to enter a value in triangle.

To make it work the modifier must calculate the amount of triangle (and must convert ngons and quads into triangles for the calculation) then divide the amount of tris entered by the total number to get the ratio to use by the modifier internally.