Make Commit Window modal

I welcome any suggestions/comments regarding the visual style. I based it off the sheets in Xcode (under *Source Control*). There a few things we can iterate on. 

  * Since we no longer have a title bar, I placed the title above the text view, you may want to change format to something else or drop it entirely.
  * The status bar could be hidden.
  * One (major) downside is that if we make it sheet, users cannot arbitrarily resize the it.

Let me know if you think it is a improvement, since if we keep changing the style/behavior of the window it could annoy users.