There were an issue in the PowerNap's main window,
that the spinner's arrows would not disappear on reaching
extremums. To solve this the commit adds a setArrowVisible
functionality to the gui::BellBaseLayout, a onValueChange
callback to gui::Spinner and employs both to the
gui::PowerNapListItem to solve the issue.
Added ThreeBox template layout type. Applied it to Pure header.
Created BellBaseLayout for most of Bell windows. Modified
SideListView to proper handle new layout with page bar on/off.
