Improvements

Here is a set of improvements to Color_Preview, Gradient_Slider and Color_Dialog widgets:

* Color_Preview: A new split color mode for comparing the current color and a reference color
* Gradient_Slider can now display gradients with custom stops
* A new Hue_Slider widget (extends Gradient_Slider)
* Color_Dialog now uses the new Hue_Slider widget. The saturation/value of the hue gradient is updated along with the other gradients.
* Color_Dialog uses the color comparison mode of Color_Preview by default
* Multiple button modes for Color_Dialog:
  * "Ok/Cancel"
  * "Ok/Apply/Cancel" for semi-live editing
  * Only "Close" for live editing
* Color_Dialog now emits colorSelected when a user clicks Ok/Apply
* Alpha channel in Color_Dialog can now be hidden
* A new Color_Dialog::showColor(QColor) slot