perf scripts python: exported-sql-viewer.py: Add ability to find symbols in the call-graph

Add a Find bar that appears at the bottom of the call-graph window.

Committer testing:

Using:

  python tools/perf/scripts/python/exported-sql-viewer.py pt_example branches calls

Using the database built in the first "Committer Testing" section in
this patch series I was able to:

  "Reports"
      "Context-Sensitive Call Graphs"
           Control+F or select "Edit" in the top menu then "Find"
                __poll<ENTER>

and find the first place where the "__poll" function appears, then
press the down arrow in the lower right corner and go to the next, etc.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181001062853.28285-15-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>