Add way to customize the panel labels.

All is said in the subject. With the following ~/.config/darktable/darktable.gtkrc I'm able to change the color of the module label. The default color is not changed.

   include "/opt/darktable/share/darktable/darktable.gtkrc"
   style "clearlooks-header-panel-label"
   {
           fg[NORMAL]      = "#8E9FC3"
   }
   widget "*.panel_label" style "clearlooks-header-panel-label"

I think this can go safely into master.