GtkColorSelector previous color handling

[Alexander Larsson <alla lysator liu se>]

I commited the color selector changes, but there are some questions i have
about the handling of the "previous" color.

When is the current color supposed to be copied to the previous color, and
who does this and how? As currently implemented in testgtk selecting OK
copies current to previous the first time, but then the previous color is
never changed.

/ Alex