Remove "standalone" option in gui_manager

[mathieu]

Back in the day when we could build a standalone application with every perspective, there was a "standalone" data member in each plugin perspective. This was used by the gui_manager to pick how to handle a perspective. Since then, another mechanism has been put in place to select the default perspective (which was originally also done through the "standalone" data member).

Since "standalone" is not longer used, we should get rid of the code that deals with it in gui_manager.