WIP: Prompting about saving session

[arhowe00]

Contributes to #129

Not ready for merge

Ran into a blocker that the slicer scene close takes over and closing the scene will wipe out the session data, so closing the slicer scene effectively makes the window impossible to show (as the session would be wiped). Currently in search of ways to make this routine show before any requests to close scene.

A separate issue will be disabling the scene save prompt altogether in the custom app.

[arhowe00]

Credit to @jcfr for noting the following:

Few options:

• See https://slicer.readthedocs.io/en/latest/developer_guide/script_repository.html#override-application-close-behavior
• Or update current action, you could grab it using code like fileCloseSceneAction = HomeWidget.menuAction("FileMenu", "FileCloseSceneAction")