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According to the wxWidgets documentation, it works. But of course, you
don't want to trust the wxWidgets documentation, particularly when it
deals with characters encoding. Or rather, you'd like to trust the
documentation, but you can't trust the actual implementation to work
until you tested it.

So lets see if it works, and revert commit it tomorrow if it sucks.

 * fix memleak
 * remove unused variable (psz_local always NULL...)
 * remove free(NULL)
 * remove expansion-unsafe wxDnDLocaleFree() macro variant
- Actually fix [14633] flawed logic

playlist.cpp, playlist.hpp: fix segfault when destroying playlist drop target (and use the wxDnD*Locale functions).
interface.cpp, wxwidgets.hpp: put courmisch's fixes to the DnD encoding issue in wx into wxDnDFromLocale and wxDnDFreeLocale

aka : merge the big wx changes from branch