Use KSharedConfig::openConfig("kmail2rc") to try and optimize away reparsing.

Of course it only helps if we're called by kmail or kontact, not that
much in standalone korganizer.

At least KIdentityManagement::thatIsMe has a cache, unlike the kde4 version
of kcalutils/incidenceformatter.cpp's thatIsMe which created an IdentityManager
instance every time (which made me look into this).

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