Creating drop shadows for button icons:
---------------------------------------
Use script-fu -> shadow -> perspective tool in Gimp
Params:
Angle: 130
Relative distance on horizon: 2.0
Relative length of shadow: 0.3
Blur radius: 3
Color: black
Opacity: 50
Check Interpolate

Creating disabled button icons:
---------------------------------------
Layers -> Colors -> Desaturate
Layers -> Colors -> Brightness/Contrast -> Set brightness to 80

