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

[Anders Engström]

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
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