It wasn't as easy as replacing it by KCheckableProxyModel though. The
CollectionSelectionProxyModel also did sorting and filtering. Therefore the
uses of the removed model got replaced by the following chain:

${SourceModel}             | The original source model
QSortFilterProxyModel      | Sorts the model by name
KColumnFilterProxyModel    | Makes sure that only the first column is shown
KCheckableFilterProxyModel | Makes the visible column(s) checkable

svn path=/trunk/KDE/kdepim/calendarsupport/; revision=1174843