[xmonad] Patch: IM layout converted to LayoutModifier

[Roman Cheplyaka]

I just pushed Ivan's patch together with my fix.

However, there is still one bug when using this modifier with Tabbed or
other layouts which are "non-pure" with respect to focus (i.e. depend on
focused window). You can observe it when you move focus from some slave
(chat) window to master (roster) window. Then currently shown chat
window will change to the first chat window. This is because currently
we don't remember, which window was focused previously.
I think this can be fixed using the ability of modifier to return
modified layout. Someone could try to fix this misbehaviour.

But this modifier behaves correctly (as far as I can see) with "pure"
layouts and it's even usable with Tabbed and other "non-pure" layouts,
so I pushed it.

-- 
Roman I. Cheplyaka (aka Feuerbach @ IRC)