layer should not get the "show" checkbox disabled based on visiblity slider state

[aseerel4c26]

Michael, while you're at it: The automatic "hide" feature of a layer is breaking my GUI expectations (which is most striking if you use the new mouse wheel slider movement - ticket #13703 ).

Steps:

1. scroll muuuuch to lower the Opacity of a layer until the slider is at the lowest position
2. (the layer automatically gets the "show" checkbox disabled)
3. scroll in the opposite direction to increase the Opacity again (undo your action in 1.)

Expected: layer gets visible again since you have undone your movement
Actual: the layer stays inactive and you need to check the box (which you've never enabled)

I guess it is this bit (and could not find a comment why it is there):

if (getRealValue() <= 0.001 && !getValueIsAdjusting()) {
	                setVisibleFlag(false);

Is there a reason for this feature? I would suggest to not automatically disable the "show" checkbox based on slider action. Thank you! :)