Add a function to get all relations of an accessible.

Add a function to get all relations of an accessible.

Add a default implementation for RelationTo.
Usually this should be empty and it is pointless to return
the parent-child releations in this function.
Instead we will have proper parent-child functions and also
remove this functionallity from navigate.

Having default implementations for these removes the
necessity to reimplement them to return Unrelated in
many places.

Change-Id: I13b7ad3256533681e5a0548ae15da98ca0d16b94
Reviewed-on: http://codereview.qt-project.org/4785
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>