Add a context menu (right-click) that provides options for copying to
clipboard, including, for trees, the ability to copy only the cell under
the mouse pointer.

Committer testing:

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

  Simply right click and pick "Copy selection", that at this point has
  just the first line, not expanded, then see what was copied by pressing
  shift+control+v on a terminal:

Call Path,Object,Count,Time (ns),Time (%),Branch Count,Branch Count (%)
▶ simple-retpolin,,,,,,

  Ditto after expanding, i.e. the selection continues to be just one
  line:

Call Path           Object   Count   Time (ns)   Time (%)   Branch Count   Branch Count (%)
▼ simple-retpolin

   Now select all the lines with the mouse and control+shift+v again:

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
      ▶ _dl_init              ld-2.28.so             1        9142        5.9            326                3.1
      ▼ _start                simple-retpoline       1        7457        4.8            182                1.7
        ▶ unknown             unknown                1         805       10.8              1                0.5
        ▶ __libc_start_main   libc-2.28.so           1        6347       85.1            179               98.4
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/20190503120828.25326-6-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>