FEATURE: allow toggling full window view mode for Todo and Month views.

Now we can toggle full window mode in the todo and month views
without having to go into the prefs dialog. However, the prefs
are synchronized accordingly.

In month view I used a QToolButton (double arrow left) which looks ok.
In todo view I used a QCheckBox adjacent to the Flat View QCheckBox.

Forgot to mention that I added a prefs dialog option for setting
the Flat View.

The prefs dialog options for this stuff are redundant now
and probably should be removed from koprefsdialog.cpp
Yes.. I think I'll do that.