panel: Also ignore hidden windows for proximity

We currently only ignore minimized windows, not windows that are
hidden for other reasons - namely on wayland windows are initially
hidden until they are placed.

This fixes a flicker in the transparent top bar on wayland when the
"position" of an unplaced window wrongly suggests the window is
overlapping the top bar.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/693

(cherry-picked from commit a0dc8dc7ef)

js/ui/panel.js

@@ -1085,6 +1085,7 @@ var Panel = new Lang.Class({
         let windows = activeWorkspace.list_windows().filter(metaWindow => {
             return metaWindow.is_on_primary_monitor() &&
                    metaWindow.showing_on_its_workspace() &&
+                   !metaWindow.is_hidden() &&
                    metaWindow.get_window_type() != Meta.WindowType.DESKTOP;
         });