FR: DrawableButton disabled opacity should be settable

Currently DrawableButton::buttonStateChanged() contains a local variable opacity and this is hardcoded to 0.4f when the button is disabled - would be good if this class has a method for setting this value, and / or this is configurable from the Button::drawDrawableButton L&F method.

Perfect example for FR: ResizableCornerComponent size should come from the look and feel - #2 by asimilon