fixes #3674 move widgets to remaining columns if column count is reduced when switching dashboard layout. Widgets are moved based on the height of the remaining columns. each widget is moved to the column with the smallest height (which changes while moving widgets)

git-svn-id: http://dev.piwik.org/svn/trunk@7769 59fd770c-687e-43c8-a1e3-f5a4ff64c105