Display selected commit details

- Add a slot to the Main Window for displaying commit details in the
  Commit Explorer
- Add a signal to the the Canvas that gets emitted by a GCommitNode when
  it's selected.
  - Connect this signal to the slot in the Main Window for displaying
    commit details.
- Refactor ATTACH_MODE constants in GConnectionLine from module to class
  level
- Refactor "author_date" attribute of Commit to "date_authored" for
  consistency with "date_committed" attribute
- Name Commit Detail widgets more sensibly