Qt4: add playlist-tree selection to simple preferences

I think tree-view on playlist looks ok (iconview/listview atleast),
so lets not make too hard to enable it.

modules/gui/qt4/components/simple_preferences.cpp

@@ -579,6 +579,7 @@ SPrefsPanel::SPrefsPanel( intf_thread_t *_p_intf, QWidget *_parent,
             CONFIG_BOOL( "qt-fs-controller", fsController );
             CONFIG_BOOL( "qt-system-tray", systrayBox );
             CONFIG_BOOL( "qt-notification", sysPop );
+            CONFIG_BOOL( "playlist-tree", treePlaylist );
             CONFIG_GENERIC_FILE( "skins2-last", File, ui.skinFileLabel,
                                  ui.fileSkin, ui.skinBrowse );
             CONFIG_BOOL( "qt-video-autoresize", resizingBox );

modules/gui/qt4/ui/sprefs_interface.ui

@@ -349,6 +349,13 @@
            </property>
           </widget>
          </item>
+         <item row="9" column="0">
+          <widget class="QCheckBox" name="treePlaylist">
+           <property name="text">
+            <string>Display playlist tree</string>
+           </property>
+          </widget>
+         </item>
          <item row="8" column="1">
           <widget class="QComboBox" name="stylesCombo"/>
          </item>