git-gui: Be more Macintosh like.

It is tradition for applications to store their about and preferences
menu options within the application menu.  This is the first menu in
the menu bar, just after the apple menu.  Apparently the way to access
this menu from Tk on Mac OS X systems is to create a special menu whose
name ends in ".apple" and place it into the menu bar.

So now if we are on Mac OS X we move our about menu and our options menu
into the application menu, like other Mac OS X applications.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>