UI|Client: Added stub for SliderWidget
Default Style: Cleanup of fonts.dei, added slider.label font
Refactor|UI|Client: Cleanup; converting ui::Margins to a vector

GuiWidgetPrivate now has a convenience method for accessing the
widget's style.

ui::Margins can be converted to an integer vector for drawing
purposes. To help with this, added xy() and zw() methods to Vector4<>.