Move public headers into separate directory

- headers that we want exposed to the users have been moved to include/
- problem-type-specific headers have its own subdirectory, e.g. instead
  of koops_frame.h there is now koops/frame.h
- the rest of the sources were adapted to this change

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>
Move "method tables" to files where the methods are defined

- added new header internal_utils.h with private declarations that don't
  fit into any other header
- stacktrace.c was split to generic_stacktrace.c and
  generic_stacktrace.h, the same was done with thread.c and frame.c.

  The generic_*.h headers contain the method table types and declaration
  for platform-agnostic functions that should not be called directly by
  the library user.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>