- Fix threading qualifier warnings

This will catch broken plugins at build-time rather than run-time.

I don't have a broken file browser, and I can't compile wxWidgets,
so no way I can test that.

This array is now usable as a reliable source of data for size
Use playlist_CurrentSize(p_playlist) to retrieve the size of the current playset.

Not tested heavily yet ...

For interaction implementers: you need to call playlist_Signal after changing the status of a dialog. Please see the diff for qt4/dialogs/interaction.cpp