Gitx dev stashes

Support for stashes (see #14 )
- [x] list stashes in sidebar
- [x] pop stash (context menu)
- [x] apply stash (context menu)
- [x] drop stash (context menu)
- [x] view diff (context menu, open in new window)
- [x] stash create via commit dialog

Hold down ALT-key to reveal 'Stash Changes' button and 'Keep Index' checkbox.