gitk: Restore some widget options whose defaults changed in Tk 8.5

gitk: Restore some widget options whose defaults changed in Tk 8.5

The default options for panedwindows in Tk 8.5 make the sash
virtually invisible -- the handle is not shown and the relief is
flat.  This puts the defaults back to showing the handle and a
raised relief on the sash, as in Tk 8.4.

This uses the option command to do this, and also uses the option
command to set the default font for various UI elements to the
UI font ("uifont").

Signed-off-by: Paul Mackerras <paulus@samba.org>