Remove invalid using directives.
Factor out VSPScreen to a separate header. GuiDevice and GroupLayout only need to depend on the abstract screen class, not on VSPScreenFLTK.
Implement DoubleSlider.
Port AwaveScreen to DoubleSlider. Fix spinbox behavior.
Port remaining screens to DoubleSlider. Disable keyboard tracking of spin boxes as this provides bad UX. VspScreenQt automatically enables update flags when widgets are connected to SetUpdateFlag.