Tip for Preserve black setting is always shown as sensitive

The setting in the Preferences > "Input/Output" > "Colour management" > "Preserve black" is always shown normal (sensitive). For UI consistency it would be better if the tooltip is set unsensitive if the the setting itself is unsensitive.

In addition, on my system (Vista 64 bit, devlibs r29, trunk r11358) cmsFLAGS_PRESERVEBLACK does not seem to be defined and hence, the setting can never be changed. In this case, it is probably better not to show the setting at all.