gitk: use config variables to define and load a theme

gitk: use config variables to define and load a theme

gitk uses themed tk, but has no capability to alter the theme defined
by Tk. While there are documented ways to install other themes, and
to make one the default, these methods are obscure at best. Instead,
let's offer two config variables:

- theme  this is the name of the theme to use, and must be available.
- themeloader - this is the full pathname of a tcl script that
  will load one or more themes into the Tk namespace.

By default, theme is set to the theme active when Tk is started, and
themeloader = {}.  These variables must be defined to something else to
have any user visible effect.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>