Make gitk work reasonably well on Cygwin.

Make gitk work reasonably well on Cygwin.

The gitk gui layout was completely broken on Cygwin. If gitk was started
without previous geometry in ~/.gitk, the user could drag the window sashes
to get a useable layout. However, if ~/.gitk existed, this was not possible
at all.

The fix was to rewrite makewindow, changing the toplevel containers and
the particular geometry information saved between sessions. Numerous bugs
in both the Cygwin and the Linux Tk versions make this a delicate
balancing act: the version here works in both but many subtle variants
are competely broken in one or the other environment.

Three user visible changes result:
1 - The viewer is fully functional under Cygwin.
2 - The search bar moves from the bottom to the top of the lower left
    pane. This was necessary to get around a layout problem on Cygwin.
3 - The window size and position is saved and restored between sessions.
    Again, this is necessary to get around a layout problem on Cygwin.

Signed-off-by: Mark Levedahl <mdl123@verizon.net>