The `group3` addition I made in a previous commit enabled one to go so
far, but then quickly hit some walls due to non-exported symbols from
`XMonad.Layout.Groups`.
This commit removes `group3`, as it would hardly be useful to anybody,
and introduces a new `ModifyX` message that allows the modifying
function to return a `Groups` layout inside the `X` monad. Here's an
example on why this is useful:
Say you have a master layout with tabbed sub-layouts, and you have
terminal windows sprinkled around these sub-layouts. You now want to
gather all of them into a single tabbed sub-layout, effectively
implementing a "tabbed terminal" (or browser, or editor, etc). With
functionality like this, `XMonad` can become a unified multi-window
application manager: one does not need tabbed browsers, terminals,
etc.
In order for this to be possible, however, the modifier function needs
to be able to query for things like the window class name with
`runQuery`, and that in turn means it has to operate inside the `X`
monad. This is only possible if `Groups` accepts the modifier
introduced in this commit.
I bet many other uses for a `ModifierX` message can be found. I have
the functionality of the example I gave implemented and working with
this change, since it was my motivation to get it done (and I must say
it is quite sweet to have tabbed window unification).
62e04de68e