Added SKUI::ImageButton

Image button with images for default and active state (mouse down) 

Methods:
ImageButton.file=
ImageButton.activestatefile=

Event:
ImageButton.on( :click ) {|control| ... }
    
example toggle image button:
@@active_toggle = false
    img_active = SKUI::ImageButton.new(css_path + "icons/off_red.png",css_path + "/icons/off_red_pushed.png")
    img_active.on( :click ) {
      if @@active_toggle == false
        img_active.file = (css_path + "/icons/on_green.png")
        img_active.activestatefile  = (css_path + "/icons/on_green_pushed.png")
        @@active_toggle = true
      else
        img_active.file = (css_path + "/icons/off_red.png")
        img_active.activestatefile  = (css_path + "/icons/off_red_pushed.png")
        @@active_toggle = false
      end
    }
    @@skui.add_control(img_active)