Introducing the rewrite of the todo view.
This todo view is separated in a model and a view following the Qt Model/View architecture.

I followed Bruno's move to the views/ subfolder (which he did for the new monthview) in order to have view-related classes grouped together in those folders.

Still todo:
* Write delegates for various fields (date edit delegate for the due date, a widget for choosing categories, ...)
* Popup-menu action partly don't work
* Popup-menu always displays all actions enabled, even if not appropriate
* Quick search works only for the summary, not for the category.
QSortFilterProxyModel has to be subclassed to add the logic for filtering categories too.
* Some options may not be appropriate any more, has to be reviewed

Known issues:
* There used to be a second instantiation of the Todo view in CalendarView (besides ViewManager). I don't know where this second instance is used (it's created with CalendarNull::self()), but it causes problems because the todo view is initialized with an empty calendar, but receives notifications about changed items (which are not in the todo view). For now, the instance in CalendarView is commented out.

Regressions:
* I removed the support to split the todo view in 3 parts. This was used to separate todo organized by others, by yourself and something else I don't remember. There are quite a few issues with this (yet alone there were tons of FIXME's and TODO's in the code regarding this feature).

Also, the usability aspect was not clear for me (what happens if you drag and drop a todo from one view to the other?). I think this feature needs some more discussion before being added again.