terminal: add wake-up/sleep logic

terminal: add wake-up/sleep logic

Similar to the UI subsystem, we also need to be aware of our current state
inside of the terminal subsystem. We can now avoid rendering graphics when
not awake. This suppresses odd warnings that occured when keeping an
application active and printing in the terminal while the terminal was in
background.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>