If the device supports it, add a checkbox for enabling automatic
warmth, and a widget for setting the hour at which warmth should be
maximal ("bedtime").
The hour can be changed through +/- buttons; holding them will change
with 30min steps. As soon as automatic warmth is enabled, the 'warmth'
cannot be changed manually anymore, so we make its widget grey, but
the current value is still displayed correctly. Changing the hour
changes warmth accordingly.
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