Detached tabs appear when the Greeter perspective is active

Steps to reproduce:

• Create a new project,
• Drag the Effects Library tab to create a new window
• Reopen Pitivi, notice the Effects Library tab window is visible when the GreeterPerspective is active. It should be hidden.

The MainWindow switches between perspectives using the show_perspective method: mainwindow.py. This method should get the list of BaseTabs widgets used by the perspective and tell them to hide/show the detached tabs. A new Perspective.find_base_tabs methods in perspective.py iterates the widgets hierarchy in the perspective's self.toplevel_widget and return the BaseTabs widgets. The BaseTabs class in tabsmanager.py could have a new set_detached_tabs_visibility method to be used by show_perspective. Also, BaseTabs should probably create the detached tabs hidden by default, meaning it should not window.show_all() in _create_window, but instead it should notebook.show_all().