[log] IDEA-117680 IDEA-116207 Fix showing Changes for multi-selection

* MainFrame: remove single-selection limitation.
* Move pre-sorting of selected rows and post-zipping of Changes to
  the single place in VcsLogGraphTable
  (zipping was made in GraphTableModel; sorting was incorrect
  in both models).
* Add javadoc for getSelectedChanges & use List instead of int[].
* Improve javadoc for zipChanges()