Tidy up and rationalise the power off and restart dialogs

The power off, restart and log out dialogs have a variety of features which, when they appear together, can look a bit messy:

I can imagine that there are also scaling issues here, if there are multiple logged in users, or multiple inhibiting apps.

I've done some design work for these dialogs, which attempts to address these issues with a more structured approach to the dialog content:

See the full mockups for more details: