qtui: plug hook callback leaks
qtui: add 'About' and 'Quit' actions.

('actionAbout' is not connected as we do not have libaudgui-qt yet.)
qtui: add playlist action placeholders
qtui: fix opening entire directories with the file dialog
qtui: implement many playlist menu actions
qtui: process PLAYLIST_UPDATE_STRUCTURE updates
qtui: update playlist tab titles as part of the culling phase
qtui: port statusbar code from gtkui
Merge pull request #25 from kaniini/master

miscillaneous qtui fixes