** Added support for widget space dynamic grid sizing by introducing
widget space sets (widget space set may contain many widget spaces that
minimize their grid size requirement and one widget spaces that
allocates all available space)
** Added minimumSize property into Widget -entity
** Removed "top level" widget spaces (before this widgets were allowed
to be placed inside widget space that contained subspaces
(left,center,right)) because placing just one widget inside maximized
widget space inside a widget space set may be used to achieve similar
results