Scrolling by page, by half-page (v2)

Hi,

Following are two patches I already sent in a different version in January, the
first one to implement scrolling by half a page, the second one to allow some
overlapping when browsing page by page. The differences with the patches I sent
previously are:
- the order of application of the patches is reversed;
- now the vertical overlap set by the user is ignored if it is greater than the
  window's height; you really need to be using a tiny window for that to happen
  but you never know.

Regards,

Fabienne