This is a convenience module in order to have less import noise. It
re-exports the following:
a) Commonly used modules in full (Data.Foldable, Data.Applicative, and
so on); though only those that play nicely with each other, so that
XMonad.Prelude can be imported unqualified without any problems.
This prevents things like `Prelude.(.)` and `Control.Category.(.)`
fighting with each other.
b) Helper functions that don't necessarily fit in any other module;
e.g., the often used abbreviation `fi = fromIntegral`.
Commits d638dc8b and a5e87e38 introduced a per-AvoidStruts-instance
strut cache that
a) didn't get initialized at startup,
b) didn't get reinitialized after layout reset and
c) didn't get updates if it wasn't the active layout, for example when
layoutHook = avoidStruts tall ||| avoidStruts (mirror tall)
a) + b) could be fixed by using the docksStartupHook introduced in
28e9f8bc, although this wasn't documented and having to call
docksStartupHook after setLayout is far from obvious.
By moving the strut cache from AvoidStruts instances to a global state,
b) and c) are fixed. One still has to invoke the docksStartupHook for
a), and this will be addressed in the next commit.
