new callback: changeView

The *changeView* callback is called **before** a view is changed (as opposite to
*viewDisplay* which is called after the view has changed). Useful for e.g.
some housekeeping operations before the new view will appear (I need it
for destroying of popovers bound to day cells or event *`<div>`* elements. If the *viewDisplay* callback would be used the element the popover is bound to is disconnected from the DOM and the code doesn't work).