Add busy property to ShellApp

Using a separate property to show when the application is busy rather
than cramming it into the state property makes the code clearer. In most
places we only care if an app is running or not, not whether it is
actually busy.

https://bugzilla.gnome.org/show_bug.cgi?id=736492

src/shell-app-system.c

@@ -335,7 +335,6 @@ _shell_app_system_notify_app_state_changed (ShellAppSystem *self,
   switch (state)
     {
     case SHELL_APP_STATE_RUNNING:
-    case SHELL_APP_STATE_BUSY:
       g_hash_table_insert (self->priv->running_apps, g_object_ref (app), NULL);
       break;
     case SHELL_APP_STATE_STARTING: