perf scripts python: exported-sql-viewer.py: Add copy to clipboard

Add support for copying to clipboard. Two menu options are added to copy the
selected rows / columns with normal spacing, or as comma-separated-values.
In the case of trees, only entire rows can be copied.

Comitter testing:

  $ python ~acme/libexec/perf-core/scripts/python/exported-sql-viewer.py ~/c/adrian.hunter/simple-retpoline.db

Select the lines, press control+C and on the same terminal,
press control+shift+V and voilà:

Call Path                           Object           Count  Time (ns)  Time (%)  Branch Count  Branch Count (%)
  ▼ 14503:14503
    ▼ _start                        ld-2.28.so           1     156267     100.0         10602             100.0
        unknown                     unknown              1       2276       1.5             1               0.0
      ▼ _dl_start                   ld-2.28.so           1     137047      87.7         10088              95.2
        ▶ unknown                   unknown              4       4127       3.0             4               0.0
          _dl_setup_hash            ld-2.28.so           1          0       0.0             1               0.0
        ▶ _dl_sysdep_start          ld-2.28.so           1     131342      95.8          9981              98.9
      ▼ _dl_init                    ld-2.28.so           1       9142       5.9           326               3.1
        ▼ call_init.part.0          ld-2.28.so           3       9133      99.9           319              97.9
          ▶ _init                   libc-2.28.so         1       6877      75.3           110              34.5
          ▶ check_stdfiles_vtables  libc-2.28.so         1         76       0.8             2               0.6
          ▶ init_cacheinfo          libc-2.28.so         1       1991      21.8           197              61.8
      ▶

 _start                      simple-retpoline     1       7457       4.8           182               1.7

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/20190503120828.25326-5-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>