DTGTK Gradient Slider: cleanup: GdkColor -> GdkRGBA
NOTE: this results in slightly different color of blendifs and in relight iop.

This is because before max value was 32768 when it should have been 65535,
see https://developer.gnome.org/gdk3/stable/gdk3-Colors.html#GdkColor
Bauhaus popup: make sure it is a square too