- remove the separate window toolbars from the history and commit views and create a new window toolbar in the repository window
- add new toolbars inside the history view
- new class to draw a gradient in the background of a view
- moved the search field from the main toolbar to the scope bar
This changes a bunch of ivars to __weak's, to avoid reference loops.
Furthermore, in PBGitHistoryController, we need to call [webView close],
otherwise some memory will never be freed?