The function PanelView::setLenght does not respect the values of m_minLength
and m_maxLength. This lead to a panel that is too short when using xmonad.
I.e. even though the panel with is set to maximise, it only is as short as
it is needed by the widgets on it.
This patch fixes this behaviour by making sure the intial value set to length
on startup is sensible given by the settings.