py: re-introduce os/app button naming, pop views on os button press

This finally bakes in some UX assumptions about the meaning of each button.

The app button (by default the left button) is for exclusive use by applications. In the menu, the app button is used to go left/right and to select an option.

The os button (by default the right button) is for exclusive use by the OS. It always adjusts the volume and allows the user to navigate back (either exit the app or go back in menus).