[xmonad] Merged functionality from X.L.ThreeColumnsMiddle into X.L.ThreeColumns (and some new features)

I had a look at X.L.ThreeColumnsMiddle and X.L.Threecolumns and noticed that they are very similar, actually the first one is based on the second one. Therefore I tried to merge them together and here is the result. This somewhat breaks the configuration syntax, but that is unavoidable. There was a difference in the way column sizes was specified, I adopted the one from the Middle-version since I can't understand why it was done as it was in the original code. I also extended the ability to specify the column size a bit by allowing negative fractions. Negative fractions will specify the size for the slave columns and the behaviour will be different when there are only two windows to manage. Enjoy. /Anders Engström