support a `shade-time' setting rather than hard-coding 0.3s

This patch implements just about enough for users to be able to customize the shader time.  It's by no means
the finished article: I'd want to support a shade-brightness setting (and shade-desaturation, from my other patch), a tween-transition setting, and to have a ui for it in gnome-tweak-tool (a prefs.js implementation).  If this patch is OK, I will go on and do the other settings; doing the UI/prefs.js is less likely to be high on my todo-list.