Rework the commit window as a sheet (document modal)

Previously, the commit window was changed from a "always on top" window to a more typical window that could be minimized or sent to the background. While this made viewing the diffs more convenient, we could easily loose track of it or at worst end up with multiple dangling commit windows. By making it document modal, this prevents dangling windows. Furthermore, since the commit window is now attached to the main project window and the file browser already has the SCM status, we hide the file list by default and dropped the action button. Users can choose to show the file list via a checkbox and the action button's menu is still available via the file list's context menu.
Remove no longer used option for the Commit Window

Originally, the "--ask" option was used in the previous commit window to change the NSTextField above the text view; however, no SCM bundles (in textmate's repos) currently uses it.
Use dictionary literals