perf scripts python: exported-sql-viewer.py: Factor out TreeWindowBase

Factor out a base class TreeWindowBase from CallGraphWindow, so that
TreeWindowBase can be reused.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lkml.kernel.org/n/tip-ifirw0c0mhkwxg6l12lk6k4p@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>