LyXToolBox: a QToolBox with minimum size management

LyXToolBox: a QToolBox with minimum size management

The purpose of this custom widget is to allow the use of a QToolBox in a limited
area. The stock QToolBox does not provide a minimum size hint that depends on
the size of the pages; it assumes that there is enough room.  This subclass sets
the minimal size of the QToolbox. Without this, the size of the QToolbox is only
determined by values in the ui file and therefore causes portability and
localisation issues. Note that the computation of the minimum size hint depends
on the minimum size hints of the page widgets. Therefore page widgets must have
a layout with layoutSizeContraint = SetMinimumSize or similar.