gitk: Fix transient windows on Win32 and MacOS

gitk: Fix transient windows on Win32 and MacOS

Transient windows cause problems on these platforms:

- On Win32 the windows appear in the top left corner
  of the screen. In order to fix it, this patch causes
  them to be explicitly centered on their parents by
  an idle handler.

- On MacOS with Tk 8.4 they appear without a title bar.
  Since it is clearly unacceptable, this patch disables
  transient on that platform.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>