Added custom css class identifier

[janbrouwer]

Gives the option to add specific styling to each control

Feel free to reject this pull request if it's not in line with your ideas for SKUI.
I wanted additional css-classes in my theme-stylesheet with specific formatting for some controls and I couldn't find a built-in method to accomplish this.

[thomthom]

Can you provide some examples to when you needed to use custom CSS classes?

[janbrouwer]

I used it for styling specific buttons ( a more general approach to create image buttons, for which I have not yet found a satisfactory solution) but this could also be used to make a specific element "pop out". Like "don't forget to click me! I have a blinking edge!" or something...
I first tried specific styling through javascript and the css-id, but that only seems to work AFTER the window is shown.
This seemed like a very simple solution to add some specific styling for an element.
But it could be that I'm missing an already working method for this. ;-)