[xmonad] darcs patch: improve XMonad.Layout.Mosaic

It seems more useful to have the list of relative sizes (the only argument to the Mosaic constructor) to represent the master window by the first element, the window just below the master as the second, until either that list runs out, or there are no more windows. Now it is possible to behave like Tall's resizing with:

Mosaic [3,1,1,1,1,1,1]

having a function that just changes the first element of the list; using for illustration (clearly the empty list case should be handled);

sendMessage (SlopeMod (\(x:xs) -> x*2:xs))

In the previous scheme, we didn't know which element of the list represented the size of the Master. -- Adam