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.
