Fade code which was disabled by preprocessor during indexing

[PathogenDavid]

When code is disabled by the preprocessor it isn't indexed by the generator, which is both understandable and unsurprising.

To help the user identify that the code isn't enabled in the configuration that was indexed, the code in disabled sections should be faded or grayed out. (Example from a similar tool for C#)

For example, see https://code.woboq.org/linux/linux/include/linux/acpi_dma.h.html#93 At a glance it seems like the code browser is broken and not working, but in reality you just need to scroll up to the enabled implementation above.