panel: Update .solid style on session mode changes

The .solid style isn't supposed to apply for modes that don't support
windows, but for this to work we have to update the style on session
mode changes.

https://bugzilla.gnome.org/show_bug.cgi?id=783202

js/ui/panel.js

@@ -1015,6 +1015,8 @@ const Panel = new Lang.Class({
         else
             Main.messageTray.bannerAlignment = Clutter.ActorAlign.CENTER;
 
+        this._updateSolidStyle();
+
         if (this._sessionStyle)
             this._removeStyleClassName(this._sessionStyle);