Make properties/values prettier in regular index about bikeshed

[tabatkins]

First half of this work is done now. Multiple index entries with the same text get collapsed into one entry with sub-entries, like:

<number>
  value for flex-shrink, 7.3.1
  value for flex-basis, 7.3.2

Doing it the other way around, where the index entry for 'flex-shrink' includes sub-entries for all of its values, is a bit more difficult, but doable. I'll get to that soonish.

[tabatkins]

Ugh, this'll actually be much harder than I thought. Problem right now is that the for='' values aren't intrinsically connected to other definitions. They're effectively just a disambiguator, with no verification that they actually mean anything right now.

I need to fix that first before I can take this issue any further.

[tabatkins]

Done. It's currently limited to properties, but the index entry will now display entries for the values of the property as well.