[ShellWindowTracker] Don't skip "uninteresting" windows for focus

For the purposes of determining which application is focused, don't
skip "uninteresting" windows.  The old get_focused_window code
was used for usage tracking, but here we want reliable application
association.

Also convert a .text= to .set_text that was missed with the last
patch.

https://bugzilla.gnome.org/show_bug.cgi?id=599206

js/ui/panel.js

@@ -146,7 +146,7 @@ AppPanelMenu.prototype = {
 
         this._iconBox.remove_all();
         this._iconBox.hide();
-        this._label.text = '';
+        this._label.set_text('');
         if (this._focusedApp != null) {
             let icon = this._focusedApp.create_icon_texture(PANEL_ICON_SIZE);
             this._iconBox.append(icon, Big.BoxPackFlags.NONE);