implement a more generic ShowList method

apt-get is displaying various lists of package names, which until now it
was building as a string before passing it to ShowList, which inserted
linebreaks at fitting points and showed a title if needed, but it never
really understood what it was working with. With the help of C++11 the
new generic knows not only what it works with, but generates the list on
the fly rather than asking for it and potentially discarding parts of
the input (= the non-default verbose display). It also doubles as a test
for how usable the CacheSets are with C++11.

(Not all callers are adapted yet.)

Git-Dch: Ignore

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