perf scripts python: exported-sql-viewer.py: Select find text when find bar is activated

perf scripts python: exported-sql-viewer.py: Select find text when find bar is activated

The user probably wants to replace the find text, so select the find
text when the find bar is activated.

That is fairly standard behaviour for search text entry.

Entering text will replace the current text, but using edit keys
(arrows, home, end etc) cancels the selection and enables editing.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190520113728.14389-23-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>