gdb: nice list walking

Introduced command 'tt-list-walk'

Changes in 'tt-list' options:
- dropped options -pre/post (previously used to implement walking
  functionality; no longer needed)
- option -filter was renamed to -predicate (so it can be specified with
  just -p while previously at least -fil need to be used to avoid
  ambiguity with -fields option)

Closes #8524

NO_DOC=gdb extension
NO_TEST=gdb extension
NO_CHANGELOG=gdb extension