Allow setting dashicon on widget in customizer

[theenoahmason]

Widget icons in the customizer are purely CSS based. They provide the unicode character for dashicons on .widget-title::before. Add a config key that will allow for adding an inline style to set this on the customizer panel.

Below is the CSS that is used to set the widget icon in the available-widgets section of the customizer:

#available-widgets [class*=text] .widget-title:before {
    content: "\f478";
}