gui/qt: add struct vlc_playlist_locker
gui/qt: add struct vlc_playlist_locker

Given that we currently do not have such entity in
include/vlc_playlist.h, this patch introduces such inside the
gui/Qt-module.

The perks of using this helper are:

    1. it makes locking/unlocking the playlist thread-safe
    2. it makes it "impossible" to forget to unlock the playlist
    3. it often makes it easier to reason about the code

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc