qtui: Use aud_get_path().
qtui: Use system icon theme on *NIX systems.
qtui: Set window icon.
qtui: Use capture-less lambdas.
qtui: Add playlist widget to .ui file.
qtui: Simplify volume button code.
qtui: Set parent object instead of deleting widgets manually.
qtui: Prevent hiding the toolbar.
qtui: Split TimeSlider class out of MainWindow.
qtui: Style fixes.  Disable another way to hide the toolbar.
qtui: Move code out of header files.
qtui: Split off dialog window code from main_window.cc.
qtui: Move actions to main_window_actions.cc.
qtui: Split off status bar code into a QStatusBar subclass.
qtui: Set window title in the same fashion as GTKUI.
qtui: Don't show "Pla" in the playback icon column.
qtui: HAVE_WINDOWS and HAVE_DARWIN are not defined in config.h, use Qt constants for this for now
qtui: playlist_tabs: allow child widgets to be deleted automatically by Qt
qtui: main window: begin removing again qt designer control.

we cannot use qt designer because it does not support our gettext-based translations.
qtui: playlist model: remove unnecessary PL_COL_PLAYING branch in header data function