[KPhotoAlbum] Thumbnail previous/next page cache preheating

[Henner Zeller]

Hi,
I am experimenting with pre-heating the thumbnail cache with the
previous/next page, so that normal paging is quite snappy, even with
slow CPU or harddisk. Now in subversion.
The preheating is only activated after some timeout of 150ms after the
user stopped scrolling so it should not slow down things for quick
scrolling but make the user interface feel much faster for normal,
slower paging. Let me know how it behaves on your machines; on my
notebook with even only one CPU it is pretty nice.

(note, the code is still experimental so there are some kDebug()'s
that are #ifdef'ed out in subversion)

cheers,
  -henner