gitk: Move hard-coded colors to .gitk

gitk: Move hard-coded colors to .gitk

The Preferences dialog gives control of the colors of some elements of
the gitk user interface, but many are hard-coded in the gitk script.
In order to allow these to be customized through the gitk config
file, these other colors are stored in variables which can be set
in the config file, thus providing a way for color schemes to be stored
and shared.

For win32, this makes the default foreground color that of window text
rather than button text.

Signed-off-by: Gauthier Östervall <gauthier@ostervall.se>
[paulus@samba.org: Reworded commit message to be clearer,
 changed filesepfgcolor to black]
Signed-off-by: Paul Mackerras <paulus@samba.org>