prenyltransferase activity terms - add missing EC/RHEA xrefs & fix some parentage

[sjm41]

(S)-3-O-geranylgeranylglyceryl phosphate synthase activity [NO XREFS]
=> add EC:2.5.1.41 & RHEA:23404

homogentisate geranylgeranyltransferase activity [NO XREFS]
=> add RHEA:38003 and EC:2.5.1.116

homogentisate phytyltransferase activity [RHEA:37975]
=> add EC:2.5.1.115

homogentisate solanesyltransferase activity [NO XREFS]
=> add EC:2.5.1.117 & RHEA:37995

(S)-2,3-di-O-geranylgeranylglyceryl phosphate synthase activity [EC:2.5.1.-]
=> remove EC:2.5.1.- and add EC:2.5.1.42 and RHEA:18109

protein geranylgeranyltransferase activity [NO XREFS]
=> add RHEA:21240

===

Also, we currently have this arrangement:

prenyltransferase activity
|__farnesyltranstransferase activity [EC:2.5.1.29, RHEA:17653]
        |__farnesyl-diphosphate farnesyltransferase activity [EC:2.5.1.21]
        |__protoheme IX farnesyltransferase activity [EC:2.5.1.141, RHEA:28070]
        |__squalene synthase activity [EC:2.5.1.21]
|__geranyltranstransferase activity [EC:2.5.1.10, RHEA:19361]
         |__(S)-2,3-di-O-geranylgeranylglyceryl phosphate synthase activity [EC:2.5.1.-]
         |__geranylgeranyl-diphosphate geranylgeranyltransferase activity [EC:2.5.1.32, RHEA:22296]

The 3 children under 'farnesyltranstransferase activity' and the 2 children under 'geranyltranstransferase activity' are not subtypes of those two terms, and should instead be direct children of 'prenyltransferase activity'