
Sliders provide a common type of control in which a numeric value
is adjusted by dragging a knob along a vertical or horizontal track.

FLTK provides 2 types of vertical and 3 types of horizontal slider:

Fl_Slider(x-position,y-position,width,height,label)
Fl_Nice_Slider(x-position,y-position,width,height,label)
Fl_Hor_Slider(x-position,y-position,width,height,label)
Fl_Hor_Nice_Slider(x-position,y-position,width,height,label)
Fl_Value_Slider(x-position,y-position,width,height,label)

The "nice" sliders simply have a slightly more detailed appearance.

Fl_Value_Slider adds a numerical output box to the slider.
It's value is floating point by default, but it can be changed to
an integer by using an integer as its step value:
my_slider:step(50)

Fl_Slider can be forced horizontal or horizontal nice:
my_slider:type(fltk.FL_HORIZONTAL)
my_slider:type(fltk.FL_HOR_NICE_SLIDER)

Fl_Value_Slider can be forced vertical or nice vertical:
my_slider:type(fltk.FL_VERTICAL)
my_slider:type(fltk.FL_VERT_NICE_SLIDER)

Callback is done each time its value changes.
