add support to select patch sets

- add new editable column to select the patch set to be displayed
  for each change in the changes list
- all actions on the changes list row are now based on the selected
  patch set (review, cherry pick, etc)
- display changes for the selected patch set in the changes browser
- display and add comments for the selected patch set in diff view
- resolves #22
improve column to select patch set in changes list

- change title to "Patch Set"
- add total number of patch sets to each entry
improve column to select patch set in changes list

- show a little down arrow in the column to better hint that a value
  can be selected in this column
Merge pull request #94 from forrert/changes-list/select-patchset

add support to select patch sets