We can now DnD a file everywhere on the playlist dialog. Droping above the tree will insert at the begining of the playlist. Droping under the tree will insert at the end of the playlist.

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

dialogs/playlist.*: Add File Drop in playlist handling. (Still needs to include courmisch's wx encoding fix when the fix gets fixed)

(as it is highly distribution dependant)
On Debian --with-x509-ca-bundle=/etc/ssl/certs/ca-certificates.crt will do
(provided you have the ca-certificates package)

modules/gui/wxwidgets/dialogs/playlist.cpp: add some more checks. Fixes infinite loop when draging an item outside the wxTreeCtrl widget.
src/playlist/item-ext.c: detach item from first matching parent node only.

TODO: merge this playlist code with the gibalou mini playlist code.

zorglub: i'm not really sure about the i_serial incrementation stuff, could you give it a look ?

- Use Debian default
  much faster, but properly only works on Debian and its derivatives

- Cosmetic