Use a light blue for module labels.

This strike-out a bit more and ends-up more readable given the number of
module in darktable at this point.

I had proposed a bold label and it was rejected. I'm using more and more darktable and I really feel that I'm having hesitation/difficulties to see where are the modules. All the text being white the different modules does not strike out enough and I loose time "parsing" the screen. At first I thought that with the habit it will be easier, but not much and after all this time using darktable this is not satisfactory for me.

So, this simple patch adds a light blue color to the module labels. Looks better to me.
Please use it, look at this and tell me what you think.