window-list: Do not pick up transparency from top bar

The top bar now uses a translucent style when no windows are in its
proximity. As translucency looks odd in some situations (in particular
with maximized windows), we don't want to pick it up unconditionally.
If someone fancies to integrate with the top bar's proximity tracking,
they are welcome to have a go, but for now we just restore the former
solid style unconditionally.

extensions/window-list/extension.js

@@ -781,7 +781,7 @@ const WindowList = new Lang.Class({
         this._monitor = monitor;
 
         this.actor = new St.Widget({ name: 'panel',
-                                     style_class: 'bottom-panel',
+                                     style_class: 'bottom-panel solid',
                                      reactive: true,
                                      track_hover: true,
                                      layout_manager: new Clutter.BinLayout()});