Log Message:
Update nxtvepg to 2.5.2

Changes since 2.5.1 (Unix changes only):
* removed German network 3sat from list of known Nextview EPG providers
* XML export via control menu dialog no longer applies current filter
  setting, i.e. all programmes of all networks in the database are
  exported.
* fixed compiler warnings for gcc 3.3 (signed/unsigned type conflicts)
* UNIX: added compiler switch -DZVBI_DECODER in the Makefile to
  replace VBI (Teletext) slicer with code from libzvbi which performs
  better on channels with weak reception; sometimes this slicer seems
  to eat up a lot of CPU (up to 25% on my 400MHz K6-2)