GtkCssProvider: Transform custom colors to a @define-color rule.

GtkCssProvider: Transform custom colors to a @define-color rule.

Given there are other rules such as @import (which will be supported),
keep a sane namespace here, so for (re)defining a color name, in the CSS
file it will look like:

@define-color color-name #fff;
@define-color other-color mix (@color-name, #f00, 0.4);