panel: Add style class for orientation to PanelWidget
panel: Add style class for orientation to PanelWidget

  Add style class "horizontal" or "vertical" based on the orientation
  of the PanelWidget. This allows themeing to adjust styling for vertical
  panels. An example selector would be:

  PanelWidget.vertical {
    background-image: -gtk-gradient (linear, right top, left top,
                                     from (shade (@dark_bg_color, 1.15)),
                                     to (shade (@dark_bg_color, 0.95)));
  }

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>

https://bugzilla.gnome.org/show_bug.cgi?id=672070

GNOME/gnome-panel,GNOME/gnome-panel