Make ShellAppMonitor and ShellAppSystem singletons, remove unused code in appDisplay

We shouldn't create multiple instances of either.  Also remove a bit
of unused code in appDisplay.

src/shell-app-monitor.c

@@ -912,3 +912,19 @@ on_enable_monitoring_key_changed (GConfClient *client,
 {
   update_enable_monitoring ((ShellAppMonitor *) monitor);
 }
+
+/**
+ * shell_app_monitor_get_default:
+ *
+ * Return Value: (transfer none): The global #ShellAppMonitor instance
+ */
+ShellAppMonitor *
+shell_app_monitor_get_default ()
+{
+  static ShellAppMonitor *instance;
+
+  if (instance == NULL)
+    instance = g_object_new (SHELL_TYPE_APP_MONITOR, NULL);
+
+  return instance;
+}

src/shell-app-monitor.h

@@ -33,7 +33,7 @@ struct _ShellAppMonitorClass
 
 GType shell_app_monitor_get_type (void) G_GNUC_CONST;
 
-ShellAppMonitor* shell_app_monitor_new(void);
+ShellAppMonitor* shell_app_monitor_get_default(void);
 
 /* Get the most popular applications for a given activity */
 GSList *shell_app_monitor_get_most_used_apps (ShellAppMonitor *monitor,

src/shell-app-system.c

@@ -299,3 +299,19 @@ shell_app_system_get_all_settings (ShellAppSystem *monitor)
 {
   return monitor->priv->cached_setting_ids;
 }
+
+/**
+ * shell_app_system_get_default:
+ *
+ * Return Value: (transfer none): The global #ShellAppSystem singleton
+ */
+ShellAppSystem *
+shell_app_system_get_default ()
+{
+  static ShellAppSystem *instance = NULL;
+
+  if (instance == NULL)
+    instance = g_object_new (SHELL_TYPE_APP_SYSTEM, NULL);
+
+  return instance;
+}

src/shell-app-system.h

@@ -29,7 +29,7 @@ struct _ShellAppSystemClass
 };
 
 GType shell_app_system_get_type (void) G_GNUC_CONST;
-ShellAppSystem* shell_app_system_new(void);
+ShellAppSystem* shell_app_system_get_default(void);
 
 GSList *shell_app_system_get_applications_for_menu (ShellAppSystem *system, const char *menu);