improve notification and error handling for review & submit action

- show message in status bar when these actions are successfully executed
- some refactoring for more intuitive exception handling in GerritUtil
  (make sure consumers/callbacks are only executed, when REST call
  was successful or no error message was provided)
- resolves #90
Merge pull request #95 from forrert/features/actions-notifications

improve notification and error handling for review & submit action
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